It's been a long time since I worked in a "wet" darkroom but I remember we had viewing filters for judging the color of a print. They came in primary colors and different "strengths" of each color. When you were trying to judge just how much "off" your print was, you'd hold up, say, a 5Magenta filter, or a 10yellow filter, in front of your print and see how it looked when viewed through the print. So the result was, when you found what filter made the print look the best, you could say that the print was off by 15 cyan (or whatever) then make that much of a correction in your printing.
So now I'm using PS Elements. When making test prints, I'm looking for a way to change the color of an image by a certain, known, repeatable amount, like 5magenta or 10yellow. I seem to remember seeing that, and it certainly seems like it would be do-able but I'm drawing a blank on it.
Does anybody know how to do that? (Or does everybody but me know how to do it?)