"Welke kleur heeft de jurk?"
Translation:What color is the dress?
Both are accepted, but no it "should" not be "is" perse as in Dutch an object typically "heeft een kleur".
In English, we distinguish between 'what' and 'which' by the number of options open to us. E.g what is your favourite colour? (many options). And which colour on the flag represents 'liberty'? (few options). Other languages don't look at it quite that way.
When they ask about categories they often use the equivalent of our 'which'.
In Portuguese, for example they would make the question 'which is your name?'
Just remember that the rules that apply to one's own language are not necessarily followed by others, although Dutch does seem more similar to English than most, thank God.