Zbrush is a pain in the a... for making textures. Nobody use materials there except on a square plane where you can displace your displacment texture and make a few material tricks. If you prefer to scultp/polypaint with materials you can use Polygroups >>> From polypaint . It's actually not only polypaint but material…
For your fungus example above I would create the spots using nanomesh, assign a polygroup and use the polygroups to polypaint feature which enables you to then bake a matID mask(based on different vertex colours) which you can then use in S Painter to easily mask out different materials. You can also retain polygroups if…