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• Color information channels are created automatically when you open a new image.
  
• The image’s color mode determines the number of color channels created. For example, an RGB image has a channel for each color (red, green, and blue) plus a composite channel used for editing the image.
  
• Alpha channels store selections as grayscale images. You can add alpha channels to create and store masks, which let you manipulate or protect parts of an image.
  
• Spot color channels specify additional plates for printing with spot color inks.
  
• An image can have up to 56 channels. All new channels have the same dimensions and number of pixels as the original image.
  
• As long as you save a file in a format supporting the image’s color mode, the color channels are preserved.
  
• Alpha channels are preserved only when you save a file in Photoshop, PDF, PICT, Pixar, TIFF, PSB, or raw formats. DCS 2.0 format preserves only spot channels. Saving in other formats may cause channel information to be discarded.
  
  

• When you select part of an image, the area that is not selected is “masked” or protected from editing. So, when you create a mask, you isolate and protect areas of an image as you apply color changes, filters, or other effects to the rest of the image. You can also use masks for complex image editing such as gradually applying color or filter effects to an image.
  
• Masks are stored in alpha channels. Masks and channels are grayscale images, so you can edit them like any other image with painting tools, editing tools and filters. Areas painted black on a mask are protected, and areas painted white are editable.
  
• To save a selection more permanently, you can store it as an alpha channel. The alpha channel stores the selection as an editable grayscale mask in the Channels palette. Once stored as an alpha channel, you can reload the selection at any time or even load it into another image.
  

Photographs, magazines and other objects of nature such as an apple; create color by subtracting or absorbing certain wavelengths of color while reflecting other wavelengths back to the viewer. This phenomenon is called subtractive color.

A red apple is a good example of subtractive color; the apple really has no color; it has no light energy of its own, it merely reflects the wavelengths of white light that cause us to see red and absorbs most of the other wavelengths which evokes the sensation of red. The viewer (or detector) can be the human eye, film in a camera or a light-sensing instrument.

The subtractive color system involves colorants and reflected light. Subtractive color starts with an object (often a substrate such as paper or canvas) that reflects light and uses colorants (such as pigments or dyes) to subtract portions of the white light illuminating an object to produce other colors. If an object reflects all the white light back to the viewer, it appears white. If an object absorbs (subtracts) all the light illuminating it, no light is reflected back to the viewer and it appears black. It is the subtractive process that allows everyday objects around us to show color.

Color paintings, color photography and all color printing processes use the subtractive process to reproduce color. In these cases, the reflective substrate is canvas (paintings) or paper (photographs, prints), which is usually white.

What is Color?

Color is all around us. It is a sensation that adds excitement and emotion to our lives. Everything from the cloths we wear, to the pictures we paint revolves around color. Without color; the world (especially RGB World) would be a much less beautiful place. Color can also be used to describe emotions; we can be red hot, feeling blue, or be green with envy.

In order to understand color we need a brief overview of light. Without light, there would be no color, and hence no RGB World. Thank God for light!

Light is made up of energy waves which are grouped together in what is called a spectrum. Light that appears white to us, such as light from the sun, is actually composed of many colors. The wavelengths of light are not colored, but produce the sensation of color.

Raster Images

• A Raster image is a collection of dots called pixels.
  
• Each pixel is a tiny colored square.
  
• When an image is scanned, the image is converted to a collection of pixels called a raster image
  
• Scanned graphics and web graphics (JPEG and GIF files) are the most common forms of raster images.
  
• The quality of an imprint produced from a raster image is dependant upon the resolution (dpi) of the raster image, the capabilities of the printing technology and whether or not the image has been scaled up.
  

Vector Images

• A vector image is a collection of connected lines and curves that produce objects.
  
• When creating a vector image in a vector illustration program, node or drawing points are inserted and lines and curves connect notes together.
  
• Each node, line and curve is defined in the drawing by the graphics software by a mathematical description.
  
• Text objects are created by connecting nodes, lines and curves.
  
• In a vector object, colors are like clothes over the top of a skeleton.
  
• They can be scaled up or down without any loss of quality.
  
• Since vector images are composed of objects not pixels, you can change the color of individual objects without worrying about individual pixels.
  

• Normal
  The default mode, this option displays every pixel in the active layer normally, regardless of the colors in the underlying layers. When you use opacity values (whether Opacity or Fill) of less than 100 percent, the color of each pixel in the active layer is averaged with the composite pixel in the layers behind it. Normal mode is called Threshold when you're working with a bitmapped or indexed-color image.
  
  
• Dissolve
  This option specifically affects feathered or softened edges. If the active layer is entirely opaque with hard edges, Dissolve has no effect. But when the edges of the layer is fade the result color is a random replacement of the pixels with the base color or the blend color, depending on the opacity at any pixel location. However, when you drop the Opacity value below 100 percent, Dissolve will dithers all pixels.Note: Dissolve does not dither pixels in the drop shadow layer effects. Layer effects are governed by their own independent blend modes.
  
  
• Darken
  Darken applies colors in the active layer only if they are darker than the corresponding pixels below for each channel and pixel-by-pixel. Pixels in underlying layer that lighter than the blend color are replaced, and pixels darker than the blend color do not change. For example in one corresponding pixels, in blue channel in the active layer darker than the blue pixel of underlying composite pixel and the red and green components lighter. In this case, Photoshop assigns the blue component but not the red or green.
  
  
• Multiply
  Looks at the color information in each channel and multiplies the base color by the blend color. The result color is always a darker color. Multiplying any color with black produces black. Multiplying any color with white leaves the color unchanged. When you're painting with a color other than black or white, successive strokes with a painting tool produce progressively darker colors. The effect is similar to drawing on the image with multiple magic markers.
  
  
• Color Burn
  Looks at the color information in each channel and darkens the base color to reflect the blend color by increasing the contrast. Blending with white produces no change. Color Burn results in crisp, often colorful, toasted edges.
  
  
• Linear Burn
  Looks at the color information in each channel and darkens the base color to reflect the blend color by decreasing the brightness. Blending with white produces no change. Linear Burn creates a smoother, less vibrant effect than Color Burn.
  
  
• Lighten
  Opposite with Darken mode, Lighten applies colors in the active layer only if they are lighter than the corresponding pixels in the underlying image. As with Darken, Photoshop compares the brightness levels of all channels in a full-color image. Pixels of underlying layer that darker than active layer are replaced, and pixels lighter than the blend color do not change.
  
  
• Screen
  Screen is the opposite of Multiply. Rather than creating a darker image, you create a lighter image. Photoshop looks at each channel's color information and multiplies the inverse of the blend and base colors. The result color is always a lighter color. Screening with black leaves the color unchanged. Screening with white produces white.Screen is useful for creating glows, retaining just the light colors in a gradient, and creating light noise effects such as snow and stars.
  
  
• Color Dodge
  Looks at the color information in each channel and brightens the base color to reflect the blend color by decreasing the contrast. Blending with black produces no change. When you apply the Color Dodge modes, each color in the layer becomes a brightness-value multiplier. Light colors such as white produce the greatest effect, and black drops away.
  
  
• Linear Dodge
  Create similar effect with Color Dodge. Looks at the color information in each channel and brightens the base color to reflect the blend color by increasing the brightness. Blending with black produces no change. Linear Dodge creates similar but smoother effect than Color Dodge.
  
  
• Overlay
  Multiplies or screens the colors, depending on the base color (underlying layer). Patterns or colors overlay the existing pixels while preserving the highlights and shadows of the base color. The base color is not replaced but is mixed with the blend color to reflect the lightness or darkness of the original color.
  
  
• Soft Light
  Darkens or lightens the colors, depending on the blend color. The effect is similar to shining a diffused spotlight on the image.If the blend color (light source) is lighter than 50% gray, the image is lightened as if it were dodged. If the blend color is darker than 50% gray, the image is darkened as if it were burned in. Painting with pure black or white produces a distinctly darker or lighter area but does not result in pure black or white.
  
  
• Hard Light
  Multiplies or screens the colors, depending on the blend color. The effect is similar to shining a harsh spotlight on the image.If the blend color (light source) is lighter than 50% gray, the image is lightened, as if it were screened. This is useful for adding highlights to an image. If the blend color is darker than 50% gray, the image is darkened, as if it were multiplied. This is useful for adding shadows to an image. Painting with pure black or white results in pure black or white.
  
  
• Vivid Light
  Burns or dodges the colors by increasing or decreasing the contrast, depending on the blend color. If the blend color (light source) is lighter than 50% gray, the image is lightened by decreasing the contrast. If the blend color is darker than 50% gray, the image is darkened by increasing the contrast.
  
  
• Linear Light
  Burns or dodges the colors by decreasing or increasing the brightness, depending on the blend color. If the blend color (light source) is lighter than 50% gray, the image is lightened by increasing the brightness. If the blend color is darker than 50% gray, the image is darkened by decreasing the brightness.
  
  
• Pin Light
  Replaces the colors, depending on the blend color. If the blend color (light source) is lighter than 50% gray, pixels darker than the blend color are replaced, and pixels lighter than the blend color do not change. If the blend color is darker than 50% gray, pixels lighter than the blend color are replaced, and pixels darker than the blend color do not change. This is useful for adding special effects to an image.
  
  
• Hard Mix
  The Hard Mix blend mode combines the pixels in your layers using the Vivid Light blend mode and then performs a color threshold operation on them. Hard Mix mixes two layers and pushes the colors to their absolute extreme. All in all, Hard Mixed pixels come in only eight colors: black, white, red, green, blue, cyan, magenta, and yellow, the end result being quite similar to the Posterize command (Image » Adjustments » Posterize).
  
  
• Difference
  Difference inverts lower layers according to the brightness values in the active layer. White inverts the composite pixels absolutely, black inverts them not at all, and the other brightness values invert them to some degree in between. Blending with white inverts the base color values; blending with black produces no change.
  
  
• Exclusion
  Creates an effect similar to Difference Mode but lower in contrast often smoother effect. Blending with white inverts the base color values. Blending with black produces no change. Exclusion sends mid-tone to gray, much as Pin Light sends mid-tone to transparent.
  
  
• Hue
  Creates a result color with the luminance and saturation of the underlying layer color with the hue of the blend color in active layer or painted color.
  
  
• Saturation
  Creates a result color with the luminance and hue of the base color and the saturation of the blend color. Painting with this mode in an area with no saturation (gray) causes no change.
  
  
• Color
  Creates a result color with the luminance of the base color and the hue and saturation of the blend color. This preserves the gray levels in the image and is useful for coloring monochrome images and for tinting color images.
  
  
• Luminosity
  Creates a result color with the hue and saturation of the base color and the luminance of the blend color. This mode creates an inverse effect from that of the Color mode.