when your tamagotchi is a baby, white means girl, black means boy. After the baby stage, color is just how the tamagotchi character looks, and characters can be arranged as good or bad, but it has nothing to do with the color of them.
on the v5, babies are black and white, but that doesnt even apply to gender. in the v5 menu you can see which babies are which gender, cuz you cant tell by color, like you can with every other version, including v6.
-Hope I helped! T~D