I really like how you paint eyes, mines are not that good. These artifacts when changing subdivs have been a huge problem for me for years. Sometimes in change the subdiv to rework the sculpt after painting, and then when I go back to the high subdiv, the polypaint is all messed up, a real pain. Painting OR blurring/smoothing can cause this, so you better deactivate RGB on your smoothing brush. Also there is another way to make a smooth mask : use the move gizmo or the transpose line holding CTRL and drag on the area you want your gradient on - like when you're posing a character ). It works on high subdivs too.
The ZColor plugin does not work on new versions of Zbrush. I found out that this may be related to the name of the main folder where zbrush is installed. Does anyone have any ideas?