"The pants are available in red and black."
Translation:Byxorna finns i rött och svart.
Rött and svart are here the names of the colours rather than adjectives describing the pants.
The dictionary form of the adjective is röd, but the dictionary form of the noun is rött. So it's the noun form here.
Good question! The reason is that this kind of det never goes together with definite nouns. The simplest way of seeing it may be by comparing how "There are the pants in the shop" doesn't work for saying what "There are pants in the shop" says.
Another way of explaining it is that det in expressions like det finns is used precisely because the pants or whatever aren't known to the listener (and this is the same reason why we don't use den in these sentences).
det finns en bok på golvet 'there's a book on the floor' – the book isn't known to the listener, so the sentence needs det as a placeholder because while en bok ligger på golvet 'a book is lying on the floor' is totally correct, it isn't a good sentence from the point of view of how we want to speak: it feels odd to start a sentence with a totally unknown book. We want to start out by informing the listener that there is a book.
boken ligger på golvet 'the book is on the floor' - when the book is known, it becomes definite and we don't need the det anymore. Since the listener knows what book I'm talking about, it's natural to let it go first in the sentence if I want to.