Etymology of Orange: The first thing to be called 'orange' was the orange tree, then the fruit, then the colour. In english 'yellowred' was what we know now as orange (colour).
Apple was a word for all fruit, which is why you have 'appelsin, äpple, in swedish, apple, pineapple, in english, pomme de terre, pomme, pomme d'épin in french. So oranges were once know as orange apples. In swedush they must never have made the switch.
apelsin is actually a borrowing from Dutch or Low German, and originally meant "Chinese apple". Most European languages have names for the orange that are either cognate with "orange", translate as "Chinese apple", or are borrowed from a language where it translated as "Chinese apple"
While it's true that "apple" was often used for other fruits, it was not the case for English "orange" (although it was in Swedish).
The name "orange" was originally used for the fruit itself rather than the color, thus it was never known as an 'orange apple' which would have been as nonsensical as saying 'lemon apple'. As oranges grew popular in Europe, most European languages reused the name of the fruit for the color they often called 'yellow-red' before.
The recommended conjugation pattern (the one suggested by SAOL, and therefore also the one we will recommend) is to always write "orange", no matter gender or amount. The only accepted alternative is "orangea" for plural and the definite form, but it will never be set as the recommended answer.
But I agree that most people would add -t or -a in spoken language.
weird, I just had a flashback: as a kid I lerned that the orange was yellow : 'en gul apelsin'. I just remembered. Maybe because 'orange' as a color was not so common back then 100+ years ago. but maybe instead; brandgul... definately is a cool word. I have not heard anyone use it for long time, though. I will start using it again !