Red simply indicates your worst tank. Once you fill up a few more times you'll see one bright green (best tank), many a darker green, and one red. Here is my example. My worst tank is above the EPA combined, but its still red since it is my worst tank.
I'd also stick with just putting in the straight EPA rating. With a bit of practice you'll be beating the highway mileage in the city and it won't matter nearly as much. Calculating every tank with X% city X% highway is getting a bit on the complex side. Some of us (me included) don't drive the entire tank of all our cars, so how are we to know the % break downs?