I think you probably want to stick with flat, as semi gloss and glossy paints will form a tighter membrane and seal too well. The flat, especially when diluted, should breath some.
Also, as far as color goes, white is best. I have several propane tanks, and one which was a silver color the others white. The silver one got hot enough to vent on warm sunny days until I painted it white like the others. Didn’t seem like silver would be all that much “darker” than white but it is.