7 fruits or food types overall including the Han Lemon and this theory has already been talked about lots of times.

Not only was this theory mentioned in The R&D Thread, but it was also made into a full forum post called The Missing Manual. This mentioned about the darkening of one colour lightens the other two and vice versa.

I tried to gather as much as this information as possible and put it together over here in my Progression to a Soot Black Chocobo thread.