According to the hanky code, wearing red signifies you have an interest in fisting. This could be a red hanky in the back pocket, a red jockstrap, red socks, a red armband, or a red kerchief worn around the neck. How a person wears their red armbands can also signify a person’s preferred role. A bottom (fistee) will wear a red armband on the right, while the top (fister) will wear a red armband on the left. Someone that is a versatile fister (both tops and bottoms) would wear red armbands on both arms.
While this gives an indication of what a person is into, it pays to check. Some guys wear red just because they like the color with many unaware of the meaning in the leather community.