Contego is the simplest shield found in HPMOR. It's so simple to cast that it was one of the first Battle Magic spells the first years learned, besides the Sleep Hex, Luminos, and Sumerian Simple Strike Hex. It is first mentioned in chapter 19 when Quirrel mentions that It is their "first defensive spell, a small shield that was the ancestor of today's Protego (canon)." It may be an ancestor to Protego but it is noticeably weaker than other shield spells such as Protego and Prismatis. Contego does hold a distinct advantage over these other shields such as Prismatis in that it can move with the user and it requires far less magic to cast. The first year armies used this shield very often to protect their regiments from being shot from afar, and would often charge forwards with these shields raised. These shields could often be taken down in a single blast by the Breaking Drill Hex, depending on the strength of the shield caster and the drill caster. Stronger spells such as Stupefy (canon) often just smashed directly through the shield as well. Contego was still consistently used in the First Year Battles even though it was weak simply for how cheap and easy it was to cast.
Contego's color is never explicitly mentioned, but it is usually described as circular. The size of the spell itself seems to be wide enough only to cover the caster's head and torso, as described in the Forest and Underwater battles. It is mentioned that Contego is not visible in the battle within Hogwarts' Lake, meaning it's either so transparent as to be invisible underwater or it's about the same hue as the water itself.