I'm 5 years older than you and that t-shirt (very nice anyway!) still makes me think of late 1990s/ early -00s so it's still not THAT old. I owned an actual IBM PS/2 XT as a kid and they were not color-coded [1]
Yep, the colors come from the PC System Design Guide [1] developed by Microsoft and Intel in the late 90s after IBM completely lost control of the PC market.
[1] http://john.ccac.rwth-aachen.de:8000/alf/ps2_76/ps2_76_2_ful...