You are trying to have two nominative cases in one sentence, which is only meaningful in constructions like "A is B" (with A B being nouns), and corresponding questions. However, a wall is certainly not a colour, even though the English case-free construction obfuscates this fact. The Russian question would be best translated as "Of which/what colour is this wall", which incidentally would also be a more logical way of asking it in English - but for some reasons that's not how this question is actually asked.
that makes sense, but why is Какого цвета in genitive, or am i wrong about the case?
You are correct about the case. As I have already mentioned, the Russian question literally corresponds to something like "Of which/what colour is this wall", and genitive is the most natural case to use in the situations where you would use "of" in English.
Thanks for your helpful explanation of the Russian. An aside: "is" has more meanings than the identity relation of course, so "the weather in London is awful" does not conflate the two. :) You're right, it does make more sense to say "of what color is this wall", but English simplifies because it can do so without ambiguity in this case, and over time, people usually simplify where they can. Back to the topic: that does shed light on why the genitive is used in Russian.
In english the question would be "what is the color of this wall, hence the use of genitive in Russian. As per your answer mentioning two nominatives, I don't see any nominative case in this phrase.
In english the question would be "what is the color of this wall, hence the use of genitive in Russian.
Except this would imply genitive for the wrong word. Your question would correspond to "Какой цвет (nominative) у этой стены (genitive)". That is not what we are discussing here. The most natural Russian question is "Какого цвета (genitive) эта стена (nominative)". What does using genitive for "цвет" in Russian have to do with "of the wall" in English? Please re-read my earlier answer.
"What is the color of this wall" is a more natural English phrasing, not necessarily a more true translation of the sentence in question. In English, we think of subjects owning their qualities, so the color belongs to the wall, and we want to know which one belongs to it. That's why usually in English the wall would be put in genetive.
I feel like when you click on the word to see the translation it should also give you the case
Quick question: where does the какого come from? What about что? Also, will we ever use this sentence?
Какого цвета = of which colour.
"Какого" is the genitive form of "какой"="which" or "what" in reference to attributes. Meantime "что"="what" when you question a subject (What is this?) or a direct object (What did you do?), not their attributes.