Its because green is a colour that is particularly troublesome for people who are colour blind. I'm not sure if the exact details, but the ban is in consideration of colour-blindness.
Edit: Just found this: https://www.bbc.co.uk/sport/football/57880378
Actually, the original "pink" kit was red. There was no pink on it. Its just the red and white squares were so small, that they merged and looked pink from a distance.