I see what you're saying, but I still think the example works - first of all because it's a different word now, and they're not speaking English in the books, so the etymology of the word in English doesn't matter all that much; and second, because the important thing is having enough of a separate concept for it to have a separate term so you can pick out that specific variety, which they apparently don't.

I mean, whether or not the second word is derived from the first, it's still a distinct enough concept to need a separate term for it (which apparently a lot of languages that don't have very many color words actually also do; instead of having words for "blue" and "green" you'd have "sky color" and "tree color" and so forth), eventually evolving into a completely different word over time.

(It IS a weirdly specific thing to pick out as important, since brandy really IS just basically a variety of wine, but ... Vlad. XD)