Mishnah for Negaim 1:13

The bright spot in a German appears as dull white, and the dull white spot in an Ethiopian appears as bright white. Rabbi Ishmael says: the children of Israel (may I be atonement for them!) are like boxwood, neither black nor white but of an intermediate shade. Rabbi Akiva says: painters have materials with which they portray figures in black, in white, and in an intermediate shade; let, therefore a paint of an intermediate shade be brought and applied around the outside of the nega, and it will then appear as on skin of intermediate shade. Rabbi Judah says: in determining the colors of negaim the law is to be lenient and not stringent; let,therefore, the negaim of the German be inspected on the color of his own body so that the law is lenient, and let that of the Ethiopian be inspected as if it were on the intermediate shade so that the law is also lenient. The sages say: both are to be treated as if the nega was on the intermediate shade.