It is my belief, that every fruit you give your Chocobo effects the colour level. Only when you pass the threshold from one colour to another do you receive the message 'new feathers'
I believe the numbers effected by the fruit are RNG within a small range.

Here are a few of my tests

Chocobo #1
Desert to Cream yellow (1 message from 4 Plum)
Cream to Bone yellow (1 message from 4 Plum)
Bone to Sylph green (1 message from 4 Plum)
Sylph to Snow White (1 message from 3 Plum)

I thought, if I simply drop the amount of green in the colour Desert yellow then I should definitely need less plums to obtain snow white right? So~!

Chocobo #2
Desert yellow to Sylph green (1 message from 3 Plum, 1 message from 1 Valfruit)

I think if we find the range that each fruit effects the colours we will be able to more efficiently obtain colours.

I did start making this a few days ago... but I have stopped because of my above theory.