The Mishnah in Negaim quoted in the Gemara Shevuos describes different levels of whiteness which correspond to the different words used to describe leprous spots - Se'es, Sapachas and Baheres.

The Gemara seems to understand that the word Sapachas means "subsidiary" since the root is the same root as sefach- "addition." Thus there are varying levels of white involved and Chazal received a tradition that one of them is the color of an eggshell membrane.

Yoel Domb