Using the black & white photo is a great way to sort by tone, I also use a sheet of red plastic, and a sheet of green plastic together, they take all the colors out and leave the tones.
I picked them up at the hobby store for a buck a piece, & keep them in a ziplock bag... when ever I need to look at something on my design wall I just hold that up the sheets are 81/2 x11 so you can look at a large area.