Here's some theorycrafting at work, but if you look at the fruit descriptions it states darken/lighten and your 6 colors are lighten/darken variants of rgb. Imagine a yellow choco starts on the RGB scale for R:255 G: 255 B:0, now feed it lighten green (Valfruit), suddenly the rgb values shift giving your bird a more orange hue i.e R:255, G: 205, B: 0. Just a thought but it would open the bird colors to the entire color spectrum barring minute values since I'm going to guess the color drops in increments.