[MWCC]

I'm with Stefan in that you should tint your sealer. Also what is very important is what type of paint you'll be spraying and the mix ratio. If you go with a base/clear and your base is a 1:1 mix (1 part base to 1 part reducer), with one gallon of base color you'll end up with 2 gallons of sprayable material. If your going with single stage at lets say a 4:1 mix then one gallon could be tight unless you tint your sealer. A good rule of thumb is you'll use about 5-7oz of paint per panel per coat on average (depending on color and paint manufacturer) so that should give you an idea of how much you'll need.

Also be sure to use a good quality paint. Cheap paints are cheap for a reason!