Animation color change.

AliPop

So I am animating in 4.10 Pro, and am stuck at a point for what I want to do. I want to have something changing colors within the animation. Specifically in this case, I want to have someone's eyes visibly shift from green to red, as part of a vampire thing. But I don't want it to be instant, I want to be able to have it progress visibly over the course of many frames. The best visual example I can give is this:

Similarly, I'd like to know how to do hair, skin, etc all the same, but I guess the process would be all the same either way. In my searching, I haven't been able to find an answer, only people saying things in response that raise more questions for me. So, if there's a tool or plugin or anything, I might need an explanation on how to use it, since I've currently never used any plugins or anything before.