Are there any easily available things I can carry with me, that I probably have already, that I could use as a rough color reference card while taking photos with a phone?
I realize that most phones will heavily process the images themselves, but if I needed to have a way to reference a specific color, it would be helpful if I had something in my pocket that I could add to the photo. For instance, would a clean US $1 bill be useful as a green reference? What else could I add to this to get other colors?
I realize that I could just get a pocket sized color reference card, but that is outside the scope of my current project.