There are two shades of white color on one's skin that signify that one is possibly sick with a spiritual leprosy called Tsara'at. Two are mentioned in the Torah as “white spot” and “raised white spot.” Two more are derived from the word “additional,” meaning auxiliary shades of white. They thus make “two that are four.”
“White spot” is white as snow; “raised white spot” is darker and is like white wool. The shade auxiliary to snow-white is that of lime of the Temple; the shade auxiliary to white wool is the color of egg membrane. The minimal size of a white spot that is considered significant is that of a lentil bean. Two primary shades combine with each for the size of the bean; auxiliary shades combine with their primary ones. This is analogous to two kings and their governors.
Yehoshuah the son of Rabbi Akiva asked, “Why not just teach that any shade that is whiter than the egg membrane is significant?” Rabbi Akiva answered him, “To test the Kohanim, who must be knowledgeable in the shades of Tsara'at, in order to judge.”
Art: Rembrandt Van Rijn - The King Uzziah Stricken with Leprosy