Sorry but you never hear English speakers say "Banana is yellow." We'd either say 'A banana is yellow', or 'Bananas are yellow." I found an independent source: https://www.englishforums.com/English/BananaIsYellow/bkqhnk/post.htm
Well, our native (British) expert very reluctantly decided that "this can work, although I really don't like it". Those rare examples in which that could possibly work would be 'generalizations with adjectives'. I understand that's a bit dubious, but well, I trust her judgement ;) As usually, the dialects may come into consideration...
Well, I answered A banana is yellow and it got marked as incorrect. That shouldn't happen. I can't remember if I tried Bananas are yellow . I think I just gave Banana is yellow to get through the exercise.
"A banana is yellow" is the main answer :/ It really should have worked...
Added "Bananas are yellow", we do accept changes to plural in some similar, general sentences, so okay.