Nouns – Is ‘Homework’ Countable?

I think the rule (or one of the rules) you're looking for here is that some countable nouns in foreign languages are uncountable in English. This is apparent in nouns such as music. However, as far as I know, there is really no solid way to figure out which words these are — you just have to memorize them.

Additionally, another rule that pertains to switching between countable and uncountable nouns is one of plurality: In English, several words are uncountable when plural but countable when singular (e.g. hair).

"What is the color of those two dogs?"

Both dogs have the same color.

"What color are those two dogs?"

Both can have a different color, but most likely one color each.

"What colors are those two dogs?"

Both dogs may have the same colors or different colors, but they have several colors each, or at least one of them does.

"What are the colors of those two dogs?"

Both dogs can have the same multiple colors or each be of (a) different color(s).

"Leaves are different shades of brown and red in the fall."

"The reds and browns of the woods in the fall."

As for the names of the colors themselves, they can be countable and uncountable. To differentiate them, you can use "shades" for instance.