How do I paint a face so that the color changes over a specific time frame?
you can change colors over time like this:
Go to the frame where you want the color change to start.
e.g. frame 20
Now hover your cursor over the color and hit "I". This enters a keyframe for the color.
Choose now the keyframe where you want to end the color animation.
e.g. frame 120:
now choose another color, hover again the cursor over the new color and press "I" again.