Bikram hates the color green and won't have it at his training. His staff supports this, so no color green anyplace in training. You can wear it on the weekend, but honestly, if it's green, just leave it home. This doesn't apply so much to patterns, as I wore my predominately yellow hoodie that has some green in the pattern upon it. Also, my little backpack had limes, lemons and oranges on it - so again, a little bit of green. If you wear green to a yoga class, you will be kicked out of that class, period.
Here's the story, as I remember it - keeping in mind I think it was late and I was tired:
Bikram's guru had a son around 12 years old; this was before Bikram started working with his guru when he was 5 years old. The guru's son was set to do a yoga demo at a festival, in the main tent. He was wearing a green shirt, which in Indian lore signifies bad luck. His mother advised him to change, he, being a near-teen, opted to wear it anyway. So, he and a friend went off to the festival for the demonstration. While in the midst of the event, a fire broke out - the guru's son got out in time, but realized his friend was not with him. He went back for the friend, and sadly, perished in the fire. Out of respect for his guru, who believed the green shirt had a hand in this inauspicious event, Bikram eschewed green at his direction from Day 1.
That's the story as I remember it. If someone interpreted it differently, please chime in. The staff will always defer to Bikram on this, so if you ask Craig or anyone, they usually will wait and let Bikram tell you as it's a very personal story for him; however, he typically tells it at each training to ~300 people, so it doesn't appear to be a secret, hence I'm sharing it here so you understand why he asks this of you if you go to training.