If you know your bettas well, choose ones who are slow to flare at reflections, timid, laid back, etc.
Color shouldn't do anything about agression, it is more a betta's personality. However, fin types can have some small impact. Bigger finned halfmoon bettas have more difficulty lugging their fins around, so they probably won't want to waste much energy going after other fish. Crowntails who have little weight in their tails also have more energy and less luggage when it comes to chasing and being agressive.