Until the age of eighteen, Highborn children have differently coloured fur.
Male Warblood are red, female are silver.
Male Nobleblood are purple, females are blue.
Crest (females only) are yellow.
It's the first, and easiest, way for parents to tell the gender of their young when they are born. Their crests start to develop at roughly two weeks, and don't stop growing until the age of eighteen. The boy in question is roughly fifteen, and the gun he is holding is only a foam dart launcher.