What do the colors of the roman helmet crest mean?

Answer 1

There was no particular meaning to the Roman helmet crests except that if they were worn at all, each unit would have to have the same color. The ordinary soldier had a feather used on his helmet for dress parades. A centurion wore his crest in order to distinguish himself in the battle line so his men could follow him. (In battle, the Roman centurions led by example) There is no consensuses on the colors used. Sources that mention helmet feathers are divided between the colors red and yellow.

Answer 2

There is no consensus because there is no record, which suggests it was either a personal choice or the color was, more likely, selected by the legate or dictated by Augustus when he reformed the legions. There is evidence of red, white, yellow, purple, black and even alternating colors like white and black. Since purple was a very expensive dye, it is thought that only the Imperial guard legion, aka The Praetorian Guard, used purple since that color was also associated with high rank (Imperial Purple) and, later on, with royalty, again because it was exceedingly expensive. Senators wore togas with a purple stripe only, for example. In the end, we can only speculate why certain transverse crests of the centurionate employed colors other than the standard Roman red. Officers, like tribunes, wore longitudinal crests. It is thought, though, that the transverse crest was singular to the centurionate for better visibility on the battlefield--they were easier to spot from the rear so a legate could better know where his centuries were, while longitudinal crests were easier to spot from the side since most tribunes were not combatants but legion administrators, the main exception being artillery officers. But again, they were also behind the lines. That's just a theory though.