Daily Gospel: Washed Robes

Then one of the elders addressed me, saying, “Who are these, clothed in white robes, and from where have they come?” I said to him, “Sir, you know.” And he said to me, “These are the ones coming out of the great tribulation. They have washed their robes and made them white in the blood of the Lamb.

 - Revelation 7:13-14, Emphasis added.

What robes are washed? Only dirty ones. But how are robes made clean by washing them in blood? How can something so red make something so white?