Sometimes images have an overall undesirable color cast, or color tint, such as a greenish cast caused by a photo being taken in some types of fluorescent lighting. You can use Photoshop to correct the color and remove an overall unwanted color cast or to correct colors in specific areas of images. Part of the color-correction process can involve changing the numbers that Photoshop uses to represent the colors to numbers that are closer to what they should be. In order to do this, you need to know how to measure color numbers and know some color number guidelines for common image content, like neutral-colored areas, blue sky, green foliage, and flesh tones.
All the colors in every Photoshop image are represented by color numbers. When you color-correct an image, you can watch the image's color numbers to help you know what kind of corrections to make. A color's numbers will change when the color mode of the image changes, even if the color looks the same to your eye.
Different color modes use different base groups of colors to make up full-color images, and those color groups use different numbering systems. You can see an image's color mode to the right of the image's name in its tab or title bar, and you can also see and/or change an image's color mode by choosing ImageMode.
For more information about color modes and the numbering ...