There are three reasons why you might need to change the color of an image:
1: To color correct an image. Images often arrive in PS less than perfect. Their lightness might not be evenly distributed across the black / whiote range and their hue values might be biased in one direction or another (e.g. too blue).
- Details of color correction in the Color Correction Workflow page.
2: To match one element with another. Matching elements with each other is one of the cornerstone practices of the compositing and sometime painting workflow. Color matching is one part of this and calls upon similar skills learned in Color Correction.
- Details of element matching in the Matching Elements Workflow page.
3: To stylize an image. The color in an image might be 'correct' but might require aesthetic augmenting (stylization). This is a broad skill set that is served by many pages in this Wiki, but if you have mastered color correction and matching then it is technically trivial.
- When doing anything in color a knowledge of Curves is essential.
- Also check out the Color as Numbers page which shows you the underlying mathematical nature of digital color.
- See also the HSB (Hue, Saturation and Brilliance) page where you will be introduced to these values as a color 'thinking space'.
- A page, yet to be written, on bad color adjustment: Bad Color Adjustment.