Since I'm a linguist, perhaps I can help resolve some of the confusion over the expression Flasche Wein. I took a few minutes to research it in a German reference grammar.
The short answer (as the moderators have said) is that this is an "appositional" use of Wein. It is not genitive nor is it an adjective. You use it when you want to make a noun phrase modify another noun phrase, which is what makes it feel like an adjective or a genitive. A linguist would say that all three (apposition, adjective, and genitive) serve the same semantic function but with different syntax.
Linguists have a special set of grammatical terms that are intended to let us describe all the languages in the world and compare them to each other. (As one of the moderators said, we'd call this a "partitive.") Each language, however, has its own way of describing itself. When you're learning a language, you want to use the terms customarily used to describe that language--not the universal terms, which are usually more complex.
I see that there are lots of rules for when to use apposition, when to use the genitive, and when to make a compound word. I expect we'll learn those as we go along. For now, I think it suffices to know that when you want to say how much you have of something (e.g. a bottle of wine), you can use apposition to combine two nouns.
Hammer's German Grammar and Usage, 5th Edition (Durrell, 2013, section 2.7, "Genitive, Von, or Apposition?")
It depends on what kind of flask you're talking about. Those balloony wine flasks are called "Flaschen" as well (or "Ballonflaschen" if you want to be more specific). The flasks you use in chemistry are called "Kolben" and the little flasks you carry with you are "Fläschchen" (little bottles) or "Feldflaschen" (field bottles). If they are flat, they are sometimes called "Flachmann" (literally "flat man").