I do believe that exposure to oxygen and heat causes tren to get darker. This would be bad news because it would mean darker = weaker (decomposed).
I'd have to do some experiments to be sure. I've heated tren to 250 for 45 min and it didn't get very dark, but it was sealed in a vial. I've left tren in a filter, on a shelf, for a week and it got quite dark.
I'm going to go with dark is bad.