no,according to my knowledge when inanimate (but plural) things are considered,then all of them are thought as just one and feminine.thus,I have to use here collective noun, تفاح، right?
should be correct selection.تفاحة
If I'm not mistaken, with other adjectives it's a pattern typical of comparatives, isn't it? (from kabiir)akbar, fem. kubraa, (from faSiiH)afHaS, fem. fuSHaa..أخضر is one of a few adjectives (mostly referring to colours or human defects, although there are some exceptions) that have a feminine formed using a different pattern, fu3laa2.