Wanting your opinions.

I was reading online that if a test is read after the time limit and there’s a gray line, or a line with no color that it is obviously an indent. BUT if you come back an hour later and your once negative test now has color where the positive should be, this it is most likely a positive that took awhile to register. Is there any truth to this? Can your test just all of a sudden have color where the positive should be an hour later or is it likely an actual positive? Yes, I know tests should be read in the correct time frame or it’s ‘invalid.’ Just wondering if anyone knows or has experience with this!