Changing color gradually

Hi.

So I've watched this tutorial 

https://www.youtube.com/watch?v=37t1_BC6wkU

But I'm trying to figure out how to change from one color to another gradually.

For example from red to green. So it goes darker and darker red, and then to darker green until it reaches bright green. and from there to another color..

Obviously without having to create 200 manual keyframes...