[SpookyZalost]

@ Guardian: Perhaps this will help? this tutorial is a variant of the one you probably used but it's for making brushes that actually change color.
hope this helps!
https://software-tips.wonderhowto.com/ho...mp-234735/

Edit: I don't make brushes since I usually just draw it out with my wacom tablet but having tried out this tutorial I can confirm it works with the latest ver of gimp in both windows and linux.