I'm not sure how you have your button set up, but I'm guessing you want to get a graphics object corresponding to the button, or a portion thereof, create a brush with the color that was selected, and then fill in a particular rectangle (your icon) with the color you want to use.
Classes that would correspond to this would be:
Graphics
Image
Brush
Pen
Rectangle
Point
All of these are in the System.Drawing namespace.
If you post some of your own code, it might be easier for people to help you.