I agree that Tulkas ought to be blond. (Though I am perfectly happy using a wig to achieve that). The guy's name literally means 'Blond Dude'
Tulukhastâz (sic - read Tulukhaštâz?) is supposedly a compound containing tulukha(n) "yellow" and (a)šata "hair of head", hence "the golden-haired". Adapted to Quenya as Tulkas. (WJ:399)
Source: http://folk.uib.no/hnohf/valarin.htm
That being said....how many of the Valar 'have' to be white? I get that Tolkien's universe seems to consist of very few people of color, and also that casting decisions tend to be super racist in the real world. I imagine we don't want to add to that, and would rather have some variety to the cast, particularly the Valar, as they could have literally any skin tone (and even change it at will!) But I do have to admit that I'm not seeing someone with a brown skin tone pulling off a golden haired wig very convincingly in my mind, so I'm immediately jumping to the blond = white guy mental image.
I'm not sure what I think about that. I mean, Marvel had Idris Elba play Heimdall the White, so I don't think it's a bad idea to consider him for Tulkas, either. But I am having trouble mentally putting a flowing blond wig on the guy, which is silly, since....alien-looking Valar needn't keep real-world complexions. But would it look silly?
Here is a sketch Jenny Dolfen did of Tulkas in 2005: