It happens sometimes and that’s the reason why one needs to let himself have a release to not face such issues. Masturbation isn‘t some addiction; if done in a limit say 2-3 times a week, there would be no issues. The human body does need a release at some point to get rid of the stored semen. The source of outlet is 1. masturbation, 2. Wet dreams, 3. release when one gets hot.
The reason for the change in the colour of your semen must be for the obvious fact that it has been stored for so long and I presume it had a very pungent smell attached to it, because that always happens when you don’t have a release frequently.
Don’t worry about it in the future; just see to that you get rid of your sexual tension at least once a week, it helps you avoid such embarrassing situations in the future. I had once experienced the same when I was abstaining from masturbation for 3 weeks owing to my exams, fortunately for me I had my brief on, if it weren’t the case, I would’ve faced a hardship.