I have two paper N-95 masks left over from sanding and painting the floors in my house a year ago. They are the kind you used to be able to buy very cheaply in packs of three or so at the hardware store. They are not pristine - at least one of them was worn towards the end of the floor sanding and they both were knocking around in a toolbox for a year and look it.
I would like to make them look a little more cheerful and less grubby and be able to quickly tell them apart. If I paint the outside lightly with diluted watercolor, will the masks still work? If watercolor is a bad idea, what might work? Fabric dye? Food coloring? Colored pencils?