Pink hair never occurs naturally in the game, which is why there's no official handling of it in the mod or portrait pack editor. Could be added, but you'd need to dye their hair then click random portrait for it to ever be selected.
If those are the exact 3 portraits available, it'd assign scores of
100
1
100
so it'd pick the first and last equally as often, about 49.75% of the time. It'd pick the middle one 0.5% of the time. Assuming those were the only portraits that matched race/age/sex. If more matched, than it gets more complicated, but basically, if some attribute matches, then it applies a weight that makes that portrait some times more likely to be selected than one that did not, those weights are defined in settings.gd and can be changed.