"Mis gafas son naranja."
In French, when the adjective is also a real object (like a real orange) there is no agreement. Maybe it's the same in Spanish as "naranja" can both be the adjective or the noun.
That's so strange! I've never heard of it. If that's the case, Duolingo should make it clear if it expects us to know it.
Is 'gafas' referring to drinking glasses or a single pair of reading glasses?