"That fish is yellow."
Translation:Tamta ryba jest żółta.
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.