For whiteness. See Numbers 12:10. Hereby God would suggest to them how soon he could weaken and destroy the hard and strong hand by which the Egyptians tyrannised over them. It might also be done to keep Moses humble and depending upon God, and to teach him and Israel to ascribe all the future miracles not to the hand of Moses, which was weak and liable to many distempers, but wholly to the Divine power and goodness.
Related Commentaries of Exodus 4:6
Exodus 4:6
6 And the LORD said furthermore unto him, Put now thine hand into thy bosom. And he put his hand into his bosom: and when he took it out, behold, his hand was leprous as snow.