I agree with Nicholas. I'm Norwegian as well and you are right about the weird casting choices in adaptations of mythology. Having a black elf being the son of white elves (in terms of skin color) doesn't make any sense.
I agree that Idris Elba was an unusual choice for Heimdall the White. That also obscures the backstory where Heimdall and Sif are meant to be related. Hogun looking different from the other two Warriors Three makes more sense, as he is from Vanaheim. Sif having dark hair rather than blond is explained as one of Loki's pranks - she *was* blond, but... Marvel has certainly jumped on the diversity-for-the-sake-of-diversity bandwagon, as we now have a female Thor and a black teenage female IronMan (in the comics).
I want to show family relationships in casting when possible. This is not a stage production where family features don't matter. I recognize that trying not to confuse the audience any worse than we already will with the cast of thousands is a consideration. So, having two characters who look *very* similar to each other means they'll be mixed up, and we might have to introduce some differences.
I mean, they clearly were worried that the audience would get Boromir and Aragorn mixed up in Lord of the Rings, and made sure to give Faramir a distinct hair color to try to get away from that.
But having a story-internal reason to back up the decisions is always good, even if story-external considerations also have merit.