I believe FIRST was responding to non-North-American teams which were having trouble finding wire that did not comply with IEC 60445 (which is described in rough terms
here).
Incidentally, this thread has prompted me to go track that standard down for another project, so if I find a copy at the library, I'll verify whether FIRST's rule is consistent with it. (If I read that linked chart correctly, our systems are unearthed DC, not negative-earthed DC, so grey, not blue should be reserved for negative. But I'd rather double-check the definitions before making any conclusion.)
I wouldn't say this is at the top of my list of rules that need to be taken out back and shot,
1 but it has issues like no exemption for custom circuits (even COTS ones like fans), and odd consequences like when someone combines +12 V and +5 V to get a +7 V output with an unusual voltage reference (they're both red wires).
1 Bumpers.