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 Strike Hex, though neither of this is ever directly shown but rather mentioned in one of Harry's thoughts about the very first battle. It seems to be a derivative of Protego based on its name, but is noticeably weaker than other shield spells such as Prismatis. Contego does hold a distinct advantage over these other shields though, in that it can move with the user, and can be casted in a much larger area due to costing less magic since it's a relatively weak shield. 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 Drill Breaker Hex, depending on the strength of the shield caster and the drill caster. Stronger spells such as Stupefy 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 vary depending on what the caster wants, since it doesn't seem to cause much strain on the caster. 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.