I ran into a problem using my color checker (below) on a photograph right beside my easel: the color that matched the photo turned out to be much darker than the color on the photo itself. My light source is a pair of floodlights at the ceiling on a 35 degree angle to my canvas (about 4' above my head when I'm sitting in front of the easel). My photo board is right next to the easel, and both it and the canvas are vertical, so the photo and the canvas get the same light. But the test surface on the color checker ends up almost perpendicular to light rays coming from the floodlights, so it is lit more intensely than either the photo or my canvas. In an outdoor setting, the light would be coming from almost the entire sky, so this problem wouldn't occur. And it also shouldn't occur if the studio lighting were diffuse. But for me to use a color checker with my lighting setup, I think its test surface would have to be vertical, like my canvas and photo board. Instead, I now use a test strip cut from a square polyethylene bottle and hold it vertically next to my photos.