Hello. First, a word of clarification: I know "soldier" may not be the most applicable term for medieval fighters, but I don't know what else to call them. I refer here to those in the army who were not men-at-arms. Also, my question is mainly concerning the high Middle Ages.
So, while actual pitched battles were clearly uncommon, they were bound to happen eventually. It is here where I'm a bit confused. Knights would wear their heraldry allowing them to identify each other in combat. But, at least until pretty late in the Middle Ages, most contemporary prices of art I've seen show the average foot soldier wearing their boring brown tunics coupled with whatever armour they happened to have scraped off the battlefield. Perhaps this is just the Age of Gunpowder buff in me, but being unable to distinguish the soldiers of two kingdoms/lords/whatever at a quick glance seems like lunacy to me. I realize that, with the absence of thick smoke from guns coat in the field, piercing bright coats are unnecessary, but was there any way to distinguish between rival footsoldiers (surcoats, painted shields, etc), or were they just packed into tight lines and told to stab in the general direction of the enemy? If there really were no distinguishing features of an army, how did could a soldier know he wasn't bludgeoning an ally to death if a line broke or during a particularly chaotic conflict a la Sluys?
The attire of common soldiers would vary immensely, obviously depending on the region and the time period involved. Soldiers contracted with a lord's retinue may have been given some kind of badge or emblem to signify their allegiance. They might also have been issued the same color and pattern of clothing or armor. Armor itself was sometimes painted. /u/waritter summarized what we know about painted armor in this comment. Besides armor, tunics and gambesons worn by soldiers would not have been uniformly brown. Medieval people liked colors just as much as people in other time periods did. Sometimes particular colors would be associated with men from a particular region. There's several references to Chesire archers wearing green-and-white tunics. Militias raised from urban centers might also have particular uniforms. The shooting guilds of Flanders were noted in some sources to have a particular style of dress which was adopted by aristocrats participating in guild archery contests. Guild soldiers were proud of their civic identity and were fiercely protective of their city's banners. Their helmets, tunics, shields, and armor could have all been decorated to reflect their identity as men of Bruges (or wherever). Uniforms weren't a standardized thing (and would not be for quite a while), but the concept of a uniform wasn't unknown to medieval soldiers. By the late middle ages, English troops would be required to bear the red cross of St. George on their tunics/gambesons/armor to identify their fellow Englishmen in the heat of battle. There's also some references to troops of various nationalities wearing white tunics as an identifier. Chronicles make it clear that banners were very important in distinguishing friend from foe on the battlefield. Accounts are replete with stories of charges aimed at a particular aristocrat's banner. In the Battle of Barnet during the War of the Roses, a case of mistaken identity between two lords' banners caused the Lancastrian line to be severely disrupted by friendly fire, which led to a Yorkist victory.