'Skin' in the Bible

So you are to say this word to them: This is the word of the Lord, the God of Israel: Every skin bottle will be full of wine; and they will say to you, Is it not quite clear to us that every skin bottle will be full of wine?

Is it possible for the skin of the Ethiopian to be changed, or the markings on the leopard? Then it might be possible for you to do good, who have been trained to do evil.