I have a lot of generational families with crazy skin tones.
Until the Ambitions expansion you would end up with one or the other, never a mixture of the two. Now you can get a mixture. I have never tried mixing red and blue though, but when my blue sims have children with regular Townies they will now be either the same as one of the parents, or a lighter colored blue.
You could always create a red and blue couple in CAS and then hit the play with genetics button when you create a child and see what happens.