Thanks nekomata, apologies for not seeing the previous post about this. I do use a basic Cintiq 16 so I will definitely try one of those changes if I can.

As a non-professional Artrage user, for me it's only an annoyance rather than a showstopper. It doesn't happen over all colours, reds seem to be more prone, blues less so, but either way, a quick press on the spacebar and move with pointer clears the marks.

I've not experienced this issue with any other graphics software on the same Mac.