Seems like you're on top of it :)
It a world with such a variety of interbreedable humanoids, it's no small wonder that many wild monster girls and bandits have crazy hair and skin colors.
With regards to "brand new ambiguous skin and hair color" it's pretty easy to check the default color ranges of a race while naming portraits. So adding one or two extra 'tags' (to those that deviate from racial norms) is not difficult on my end.
I'll leave the math that allows these hybrids a fair chance at appearing to you, and start looking at the race.gd and existing portraits packs to determine which races are the sluttiest have the most deviation from racial norms.