I use my camera in B&W mode to see values instead. I know some people have been hassled for taking pictures in stores but so far nobody has ever seemed disturbed by me doing this. I don't even usually take a picture, just look at the screen in B&W. (Maybe they can just tell what I'm doing? It's probably pretty obvious since I'll be stacking and re-stacking bolts of fabric between looks...)
I have the green and the red value finder sheets, but I rarely use them at home and I never have them with me in stores. But I almost always have my camera. If it's my own fabric at home I do usually take pictures and then look at them on my monitor so it's easier to see. In a shop, I usually just look at the screen.