What case is żółta here?
Shouldn't the 'jest' be followed by the instrumental singular 'żółtą' here? Is this some kind of exception to the rule?
it is a different rule.
if it's just adjective after jest, it is in Nominative. if it is a noun , or adjective/pronoun/quantifier etc + noun it is in instrumental.
I think I was marked down because of the missing period. Come on, Duolingo!