Checking your value. We all know that it is much easier to do with a black and white painting, as with color. If you work on a portrait, it is sometimes difficult to see if the balance is good. To check it you can take a black and white photo of your work. But there is a faster way. I have taken 2 old 3D glasses Green / Red. And made 1 green glasses and 1 red one. It works simply the green filters the red out of your portrait and makes it almost black and white. The red glasses are good for grass trees and so on.