Received a PM asking me to respond. Generally speaking, noun modifiers should touch the nouns they modify. There are some very rare exceptions to the rule (it seems like there are always rare exceptions to every grammar rule, doesn't it? ) - but for the most part, they should touch.

who (or whose) indicates a noun modifier, and when we see a "comma who" (or "comma whose") setup, the noun before the comma should be the noun that is being modified.

In the car sentence example, there is something a bit more complicated going on. From a technical standpoint, "which can be cool and trendy" modifies colors, the noun preceding the comma. But we could also describe cars as "cool and trendy" right? The explanation in the old book probably could've elaborated a bit more, but the idea is: if someone says this sentence, do you know whether the person is trying to say "cars can be cool and trendy" or "colors can be cool and trendy"? I wouldn't really be sure, unless I knew the person was a total grammar nut and would make sure to construct anything they say perfectly correctly. So, in that sense, it's ambiguous, and that's why we have to be careful about how we construct these things. But, as far as strict grammar is concerned, the sentence is saying "colors can be cool and trendy."

By the same token, in the OG problem here (which I'm only going to mention VERY briefly, because we're not supposed to discuss OG problems online!), the "comma whose" answers point to the word before the comma, but that word doesn't make sense... so those choices are all wrong. A "comma preposition" set-up, however, is more loose - it's not a strict noun modifier and doesn't have to modify only the noun immediately preceding the comma. It can modify the whole clause. So "comma with" is the way to go on this one.