Midrash su Levitico 13:8

14) (Vayikra 13:8) ("And the Cohein shall see: If the mispachath has spread in the skin, then the Cohein shall declare him unclean. It (the mispachath) is leprosy.") This comes to teach us about the spreading that it is tamei only with four appearances, those with which the um (the original spot) becomes tamei.

15) And does this not follow, viz.: The um becomes tamei and the spreading becomes tamei. Just as the um becomes tamei only with four appearances, so the spreading becomes tamei only with four appearances.

16) — But why not go in this direction? viz.: White hair is a sign of tumah, and spreading is a sign of tumah. Just as white hair is tamei with all appearances of white, (even those below the brightness of the four appearances), so spreading is tamei with all appearances of white!