What does the colored square after my equipment's name mean?

In my equipment list some, but not all, items have a colored square after their name, e.g. "Leather Helm S" has a brown square, "Yukumo Sky Kote" has a red square, but "Leather Vest" has no square.

What does the square, and its different colors, mean?

High and G Rank armors can be recoloured, as indicated by the square next to the equipment. The colour indicates what colour it currently is. You can recolour the equipment via your equipment box, by going to manage appearance > change armor pigment. Note though, that this doesn't change the entire colour scheme of the armor.