Well, it may have been tied to another gene combination that led them to not die before childbirth more often. Humans being able to perceive full color means that a predator's pattern camouflage won't fool your eyes as effectively. Or, if you wanna go old school on gender roles in primitive societies, it meant they could better perceive amongst the forest foliage the fruits, nuts and mushrooms to pick and fresh kills to steal more often, meaning they could support more kids. As to why men are more likely to be colorblind, I just can't imagine. All the great apes and probably all of our prehuman ancestors had color vision. Being social creatures, and the meme that men hunt in groups as a team would maybe negate the disadvantage of color blindness. Maybe all the guys who became master flint knappers and tool makers were the color blind dudes? I've heard that colorblindness can sharpen your skills in seeing contrast and texture, very useful for a craftsman. A guy hanging around the camp, always seen working at a skilled trade would be a naturally attractive mate for a lot of women.