You can keyframe any color. Just move to the time where you want the change to happen, hover over the color field and hit I. It'll get a yellow outline. You can keyframe almost anything in Blender. Not everything, but close. (Remember to set a key frame on frame 1 so the original color is kept.)
The only other thing you need to do is to go into the Graph Editor and set the interpolation to constant. That will make sure the color will stay the same until the next key frame. Otherwise the colors will slowly blend into each other.
Just select the node with the keyframed color and the curves will show up in the Graph Editor. There you can select constant interpolation with T, SHIFT-E or the menu (Channel >> Extrapolation Mode)