Some theories dont match my results. Here is an example... (Pay attention to the changes that only used 1 berry.)

NOTE: The number of berries listed below is how many it took to reach the "new feathers" message

Desert Yellow > 3 berries > UI Brown

UI Brown > 5 berries > Gobbiebag Brown

Gobbiebag Brown > 1 berry > Aldgoat Brown
R 180 > R 156
G 158 > G 128
B 130 > B 085

Aldgoat Brown > 1 berry > Goobbue Grey
R 156 > R 130
G 128 > G 128
B 085 > B 125

Goobbue Grey > 1 berry > Lilac Purple
R 130 > R 124
G 128 > G 098
B 125 > B 098

Lilac Purple > 3 berries > Lavender Blue

Lavender Blue > 1 berry > Plum Purple
R 128 > R 114
G 120 > G 075
B 169 > B 101

From my tests, 1 berry changes the RGB color much more than 3 to 6 in some cases. In other cases it doesnt change part of the RGB at all. Also if you look at the last example (Lavender Blue to Plum Purple), feeding it 1 berry (which should add/strengthen blue) actually LOWERS the blue in the RGB color code.