Hmmm...I once went to Calumet in NYC to buy the viewing filter they were offering. Interestingly, the salesman talked me out of it. He suggested using a viewing card such as has been described already and reminded me to keep one eye closed to diminish the 3 dimensional view two eyes offer. He also suggested that squinting a bit would help since detail would be lessened but 'gross' contrasts would be more easily discerned. He was right. I've used such procedures since and been quite satisfied with the result.

Additionally, a good excercise in dealing with color and value is to get a hold of a whole bunch of paint chip cards in a wide variety of colors from Home Depot or other paint dealer. Cut them up into individual chips and try sorting them into groups of similar values. Training your eye to recognize values despite their chroma helps one to do what the filter is used to do.