An idea came to mind -- the coronavirus antigen detection mask. The color of the mask is white when not in use. After wearing it, it turns blue if it is negative, and it turns red if it is positive. I'm not a professional, don't know if it's theoretically possible?
I thought about this too. My thinking was that it should have a replaceable pad that's the primary collector. I also wondered about a urine based test that changes color if it's positive.