From what I've seen, it's not very common for a Sim to inherit any genetics from their grandparent. I hadn't seen it happen in my game up until tonight. Excuse the photo of Gunther Munch, he died before I could get a proper photo of him so I just grabbed his screenshot off the net... But it's not just me thinking that his grandson Declan has inherited his skintone? Declan does look paler than the screenshot of Gunther, but I think it's just the photo. I don't see where his skin colour could have come from otherwise.
Also does anyone think that Gerard's purple skin is slightly lighter than his mother's? Can that happen with Aliens?

Yes, grandchildren can definitely inherit their grandparents features. My self-sim grandson has my self-sims nose, her husband's eyes and the other grandfather's mouth... in fact I'd say its very common. I will gladly share a family tree picture. (Also the grandchildren's inherited features may not become obvious to you until they are teens)

Also, in the case of a opposite-gendered child, if a girl inherited her grandfathers eyes, it would be coming through one of her parents who also have the eyes, but the girl's version of the male eyes will look a little different. For EVERY male part in CAS there is a corresponding female part which is always the same part. It explains why and keeps the children within a family all looking like actual siblings.