First of all, you either exaggerate, are very (un)lucky, or have very small sample size. Because you know, one color is always locked in Pandora, and sometimes this color is green.
Now for the topic. I’m mostly playing practice runs, so I see situation only for a slice from 0-1 to 2-0 brackets (if some color is significantly stronger than others, than you’ll face more decks of that color at higher win brackets, and will eventually play more games against that color). For five latest practice runs (18 games) I have the following statistics:
monoR - 4
RY and RG - 3
monoG, monoB, GB, GY - 2
Or, counting number of decks in wich each color had occured:
R - 10
G - 9
B - 4
Y - 5
For more statistics - join the “How was your (practice) pandora run today?” thread.