Stop motion. Inspire a newbie.

Question from a newbie:

I am doing some stop motion and about to make a short sequence with a decorator painting a wall with a roller. For each roll up and down the colour should get more and more saturated until the colour of the roller is matched. 

I have tried a few different things but none create good results and workflow is not elegant to put it mildly.

Thought using green screen would do the trick but can't get my head around how to do it somewhat elegantly.

Any ideas or pointers much appreciated.