You don't enter either your city or your highway, you enter both and your combined number.
Also, red versus green has nothing to do with the EPA, you get red for your worst tank, green for normal, and light green for your best. You can look at my fuel log to confirm.
Anyway, marking percents for each of the different types of driving is a nice idea, but it won't happen primarily because it's not something everyone can do and we want to keep the methods consistent and simple.
I do like your idea of marking techniques used though, it's something I hope to integrate soon!
EDIT: Tim beat me to the punch