Under traditional Irish law, there was no significance to any particular color, but the number of different colors you could wear simultaneously was determined by your status.

The general phenomenon of legal protections on status signifiers goes under the name "sumptuary laws". https://en.wikipedia.org/wiki/Sumptuary_law

Interesting. Does that mean the Yellow Turbans choose Yellow to demonstrate a lack of legitimacy for the Eastern Han emperor?

AFAICT the imperial color requirement came about a few hundred years later.