Similar to the hair, I will approach a layering of dark to light layering that will bring out dimension and roundness to the tail. My goal is to help you achieve weight and roundness to your subjects that makes them feel 3-D.

Paint for Background: Medium grey of your choice in acrylic. Choose warm instead of cool grey. However, be sure not to have too much magenta in the base of the grey. This chart is a great color source for greys. I like the Flannel Grey and Natural Grey.