You should probably get at least a whole gallon of color for two sprayable. Some reds do not cover well, whatever color you choose, get a sprayout card from your jobber to determine how many coats will be needed. It's much nicer to have some left than to run out, well worth the extra money, imo.
I've had jobs where I tried to calculate the amount a bit too miserly, and got to where I was gonna end up short on some of the final parts. In that case, I was able to make more and use it as a ground coat and use the original mix for the final coats. That's one thing about getting paint mixed, it's almost guaranteed to be just a smidge different each time it's made due to a variety of factors. So you always want to have some of that original mix left for repairs, etc., just makes life easier.