I think you guys are over thinking it... The fruits say they lighten a particular color. The fact that the other two colors become more prominent is because you are REMOVING one color, i seriously doubt the fruit secretly adds the other two colors, its just allowing the other two to been expressed more. I believe this because i had a Coral Pink Chocobo, and was able to turn it into a Rose Pink one using the Doman plums. It simply took away the red, making it more pale, thus creating Rose Pink. It didnt turn my choco blueish green, it made it a lighter shade of pink. If you don't believe me here are pictures.
Coral Pink


To Rose Pink


I also believe it isn't using the RGB scale everyone is going off of. Simply because you use the lightening fruits to get white, if it was by RGB logic would dictate if you reached 0, or close to it, you would get black, but you don't. It seems like the game goes off subtractive color rules AKA CMYK, which would make sense since its the color, or ink in the fruits that cause it to change color. RGB uses Additive color which is based off the mixing of light, not dye or ink.