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 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.