"oranżowy" exists in Polish, but it's pretty rare, I don't think it should be accepted here as you'd likely would be corrected. It's quite possible that many Polish people don't even know that it's a word.
It's in nominative. After the verb "być" (jest) you use instrumental if it's a NOUN is NOUN phrase (where NOUN is a noun or a noun phrase). If it's NOUN is ADJECTIVE, you use nominative instead.