(...)
I don't know much about vision in snakes, but I do know a little about vision because some primates (like us) are very rare in the mammalian world in being trichromats, and being able to distinguish reds from greens. In some primates, only the females are trichromats, not males. There are some human females (about 15-20%) who are even tetrachromats, which means they have two slightly different kinds of red-seeing cones, and can distinguish more red in the spectrum than the rest of us. Those are the people who call something is purple when it looks blue to you. Only women can be tetrachromats, though, because the Y chromosome doesn't carry as many color alleles as the X. Maybe next week I'll look into it more, because it's something I find really interesting, but I don't have time this week.