the troops in the Legions generally had to buy their own clothing, so aside from a general requirement in terms of type of clothing (tunic, belt, baldric, trousers in colder climes, scarf, cloak, etc) the soldiers basically bought what they could get. White and various shades of red* were common because red dye could be obtained fairly cheap by fabric makers, and most of the wool cloth would have been off-white to begin with. but other colors are recorded. blue seems to have been popular with troops assigned as Marines aboard naval vessels, and green, yellow, even black** appear in descriptions. Purple (Tyrian purple) was expensive because it required rare ingredients to make the dye, one reason it became a symbol of wealth and political power. though less rich imitations existed using blends of red and blue dyes, and later Indigo imported from India, which while still pricier than the red+blue double dyed imitators, still wasn’t quite as rich a color as the Tyrian Purple.
but since i’m not aiming for perfect accuracy but rather interesting visuals, i’m giving them a uniform color scheme anyway.
*including such things as Salmon and Pink… though darker colors probably appealed to the soldiers.
**not true black, but usually a blue dyed so dark it might as well be black.