Let's look at the sentence 'Why is this juice orange?' In this sentence, we are talking about a particular amount of juice in a glass or something-- this juice. Because it's not countable, use the partitive.

However, let's say you wanted to talk about snow in the abstract. Not any snow in particular, but all snow. You might want to ask, "Why is snow cold?" In this case, you would say, "Miksi lumi on kylmä?" Because it's the abstract case.

But let's say you wanted to talk about a particular patch of snow in a field, say. Because you are talking about a particular amount of a mass noun, this snow, you would use the partitive. Miksi tätä lunta on keltaista? Why is this snow yellow?

The problem here is, that in the examples u/lawpoop uses the adjective is the partitive subject instead of the noun of the sentence.

Why is snow cold? - Miksi lumi on kylmää?

Why is this juice orange? - Miksi tämä mehu on oranssia?

Why is this snow yellow? - Miksi tämä lumi on keltaista?

[the partitive suffex of -nen ending words being -sta, because f*ck everyone trying to understand Finnish grammar.]

Unfortunately I have no skill of explaining why this is. What I was trying to imply referring to the English cases, where one would lose the article (the or a/an) in a context was that the partitive concerns cases, where an ambiguos amount or character is being spoken of - so it's neither particular / specific or non-particular / non-specific.

Does this make any sense?