The tail belonging to each individual 'Paintbrush Dragon' has a different color on it.
It secretes out a different 'oil' produced by the body that 'highlights' the fur and they use it as a kind of...territorial marker
It works well since it looks like paint :3
Each hue is slightly different and these special kinds of dragons have very acute vision when it comes to colors so they can easily make out which color is what, since there are no two colors the same...or might even be a combination of colors!

If you have any other questions feel free to send mydnytedragon over at FurAffinity a note to ask