The Color Nodes
This is such a large topic that it deserves an entire Wiki devoted to it. However...
Color and Nuke
Why Nuke is and isn't a great color manipulator.
Really important nodes are marked with a asterix:
Newbies are advised to read these in the order in which they are presented.
All color edits involve one or more discreet operations that are, at heart, entirely mathematical. This is a list of them and their 'visualisation' derived from a Sampler node: Visualizing Color Operations Using the Sampler Node
This is a table of which color operations are to be found in which Node: Color Operations and Nuke's color nodes
An intro to the common interface elements of the color nodes: The Color Node Interface
When doing a color manipulation on an image it is wise to seek feedback on its color values. Heres how: Getting Visual Feedback on color manipulations
Color management blues? The cure: Color Management