Rashi on Leviticus 13:20/רש"י על ויקרא י״ג:כ׳

1מראה שפל IT BE IN SIGHT LOWER [THAN THE SKIN] — it itself is not deeper, but on account of its whiteness it appears lower and deeper ‎(מראה שפל), just as the colour of anything illuminated by the sun is deeper than the shadow (see Rashi v. 3).