Most of the plumbing washers are black but salmon, baby blue and reddish are easily found.
The washers can be flat or beveled, come in different sizes and some of them will buff to a luster easily with a small rag.
Using these washers will require a longer screw to make up for the added depth.
They can be used singly or as a stack of multiple sizes for a pyramidal effect.

Electrical Tip:
These rubber plumbing washers can also be extensively beneficial when used to bring forward a light switch or a plug in the wall so that they can be as flush as possible to the wall surface to receive the decorative plate.
This problem usually happens when the existing wall gets a “new face” and a new thickness added onto it, thus leaving the electrical boxes further back.
Replace the screws on the outlet with longer ones and impale the washers as needed.
It works!