What is meant by "white" in His, Exalted be He, saying: (Insert your hand into your bosom, and it will come forth white, without blemish), and is it a miracle?

Moses' hand emerging white and luminous without blemish was a miracle for him, a proof and "another sign," as stated in His Almighty's saying: "And place your hand over your breast; it will emerge white without blemish—[being] another sign."