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The different colors are used so that everyone isn't the same color (it would look super confusing). The way for determining color is by using an algorithm based on the user's name, so colors can't be chosen by users themselves. A simple question like this belongs in the SQSA Thread.
Also, the only way to change your colour is if someone does it for you. Yet only Staff, and those who help with the git get to change their colour. So not regs.