"I wear orange pants."
English in the US:
"pants" (trousers) = ズボン (trousers)
"underpants" = パンツ (pants)
English here in England:
"pants" = パンツ (pants)
"pantsu" is usually used for underwear. It can be used for pants though.
From an Englishman's point of view, what you wrote in your comment is rather confusing! :D
In Japanese you have different verbs for wearing different articles of clothing based on where they are on your body.
So you: をかぶります。Items on your head. をきます。Items on your torso. をはきます。Items below the waist.
There may be more granularity there, but that’s for someone better skilled than I am at Japanese to clarify.
You try saying "trousers" in America where the word is considered dated or obsolete. Pants originally means trousers, just a vulgar or commercial term for it. I don't know where people got the idea that it is limited to underwear. That's equally absurd; it was surely a misunderstanding.
I thought the word "pants" came from pantalone, pantaloons, pantalon, pantalones, pantaloni, pantalons, etc. The words Europeans, including the British, were all using for trousers in the 1700-1800's. I think the words "trousers" and "pants" only started being used more frequently than "pantaloons" in the 1830's to 1850's. It's not a misunderstanding though, in the UK if you talk about pants, everyone will think you mean underwear!!! :)
You can't always add い to a noun like that to make it an adjective. "Color" (iro/色) is a noun. The colors that do end in い-- black, white, blue, red--are already in acceptable adjectival forms. Yellow is a weird exception where the way they made it adjectival is to add an い to 色, but it seems to be an outlier. To make a noun an adjective, for pink and orange at least, you have to use the particle no の. It would be like hyphenating in English: the pink-colored shirt, the orange-colored skirt. (English doesn't have the same problem, though, of treating colors like orange and pink differently from other colors. I think they're relatively late additions to the Japanese language. )