Sirius draws on himself when he’s nervous. Little doodles, rainbows, anything and everything. It gets worse when test time comes up, and it isn’t uncommon to see him covered in a rainbow of colours around finals. When offered a quill in Azkaban, he continues this, until he can’t take it anymore and the quill breaks. Once out of Azkaban and living in Grimmauld Place, he surprises the entire Order when he walks into a meeting looking like a pair of children had gotten into paint and just covered him with it.
I generated some random points (positions, colors) and to each pixel I assigned the color of the closest point. Then, I realized that Voronoi did it before but his diagram seemed way more complicated ;)