The Color Nodes
This is such a large topic that it deserves an entire Wiki devoted to it. However...
Really important nodes are marked with an asterix:
Math sub menu
- Multiply *
In this page basic color operations are visualised. These operations are related to specific nodes.
The color picker
All color nodes feature a color picker, using which a color may be selected. It is described here.
A short review of color grading here.
When doing a color manipulation on an image it is wise to seek feedback on what you are doing. Nuke has many tools with which this can be done. Here's how.
Color and Nuke
Why Nuke is and isn't a great color manipulator: Color in Nuke: an Intro