Testing a color seemingly ON the painting before adding it.
You can place a piece of clear acrylic over your dry but unfinished painting to test a colour or value before actually applying it to the painting. The product I have is called Grafix...a lightweight sheet of acrylic that is water-friendly and wipes off cleanly. I purchased it at Curry's in Toronto. It comes in sheets of 9"x12", or 22.5cmx30cm. Great way to test some of those rich darks before committing them to the painting.