Once I achieved this, I tried to get a dark tone similar to what most monkeys have. The ability to layer masks in the texturing system came in handy when I wanted to do this. To start, I created a layer with a dark brown color, layered a series of grunge maps from the Library, and set their Blending to either Subtract or Add. This gave the impression of old scars and wounds on the skin. I also took some time to paint this detail by hand using paint layers.
Over this, I worked on roughness detail. Starting with a base layer with only the Roughness map active, I hand-painted greasy areas and layered some grunge masks to create organic detail. To generate more surface information, I added some skin materials from the Library, layered some red fill layers with the albedo and roughness maps active, and manually added a cavity map to the Mask Map slot in a Fill Mask. This was done to fake micro ambient occlusion detail such as pores and wrinkles. Alternatively, you could load a curvature input map, add a curvature processor layer, and turn the Edge Intensity to 0 in Layer Settings.