Nice animation! I have been trying running animations a lot this week, 6 frame ones as well but none were close to what you just made :D

Anyway, the coloring: The white faces work fine, but I suppose the green faces are for running towards the other direction?
3 reasons for not doing that:
If you are going to have a whitish character, you can make him stand out on the background very easily, just have a dark one! There is nothing easier to see than a white character on a black background! However, if when you run towards the other side your character turns dark green (for the most part), it is hardly visible. Now I see that the face is not even 50% covered, yet you should think about this. Using very bright green could fix the problem as well
The second reason is simply that you save yourself a lot of work not making the animations different for both sides
The third is that there are a lot of games where sprites flip, even though things are not right anymore (most common: equipment switches from left hand to right hand). Probably your players wont notice that quickly either.

I also think it is more pleasant to look at a completely white version, but you probably had a reason for adding the green half :)
Good luck on it!