I have a theory for the Night Lights (Hybrids as you referred to) and why they are both not 1 color!
So you know how Toothless is an Alpha, able to control the dragons (Similar if not the same as the Bewilderbeast) … His offspring can’t be one color because several of these possible Night Fury’s welding the Alpha Abilty could be catastrophic, so my theory is that only 1 Night Fury can only exist at once! So when Toothless passes, another Night Fury could be born.
( These Offspring only appeared half white from their mother obviously )
I find it sad that Toothless never actual found a Night Fury for some reason.
Thats another Truth though, Toothless is the only of his kind, Everyone says that Toothless isn’t the only one due to the appearance of this Light Fury, but in truth, she is only a subspecies, she is NOT a Nightfury, and neither are his children. He is the only Night Fury, but I guess he isn’t the last of his actual kind since the Light Fury is only a subspecies, I mean their still Furies which still makes them the same (Minus the appearense)
And for the Light Fury, I believe she is unique compared to the others in comparison. The Light Fury has a mixed texture of white, cream, and pink as her hide, but the other Lightfuries didn’t! (If you were paying attention in the movie, there were several Light Fury’s in the background in the Hidden World) and they looked plain White, so I believe she might be like a royal blood in a way. So there couldn’t be one color of her either, otherwise it could wield that crazy blast we saw at the beginning when Toothless found her and she threw a plasma blast at Hiccup and Astrid.
That was insanely strong, she could be just as more powerful as Toothless in a crazy twisted way!!
That’s my theory of why their offspring might be both colors, so their powers can stay bound!