It's not a matter of being eye catching or remarkable, it is realistic for certain factions to have colorful soldiers. However, correctly so, many soldiers are drab and not colorful, but that doesn't mean all of them are. Even the colorful soldiers shouldn't have too bright of colors, simply because it'd look a bit cartoonish, fabric gets dirty and darker, but they'd still be fairly colorful. However, those would be exceptional soldiers, the Gauls used a lot of colors, the Britons used plenty (they both had copious amounts of dyes, and a great deal of knowledge on how to extract and make them), but most factions would employ simpler colors, and simple paints and dyes.