Trying to remake this, how do I do that? (Brush tool?)

[arinonsalvia]

Hey,
hopefully this is the correct way to do this.
I am currently remaking an irl gearset for a videogame, but now I am stuck.
As you can see in the pictures I am talking about those purple patterns, which I guess is from a paint brush. What I don't seem to be able to create is the fact that all the paint patterns are all in different shapes and with a different level of purple to them. They seem organic. I am not that familar with the brush tool and my search for finding tutorials which can teach me to do something similar has failed.

Do anyone know of any tips or tutorials?
I am grateful for any help.

[didiermazier]

If you use the brush tool you can specify variation of size and dispersion