Jer 13:18 Say unto the king and to the queen, Humble yourselves, sit down: for your principalities shall come down, even the crown of your glory.
19 The cities of the south shall be shut up, and none shall open them: Judah shall be carried away captive all of it, it shall be wholly carried away captive.
20 Lift up your eyes, and behold them that come from the north: where is the flock that was given thee, thy beautiful flock?
21 What wilt thou say when he shall punish thee? for thou hast taught them to be captains, and as chief over thee: shall not sorrows take thee, as a woman in travail?
22 And if thou say in thine heart, Wherefore come these things upon me? For the greatness of thine iniquity are thy skirts discovered, and thy heels made bare.
23 Can the Ethiopian change his skin, or the leopard his spots? then may ye also do good, that are accustomed to do evil.
24 Therefore will I scatter them as the stubble that passeth away by the wind of the wilderness.
25 This is thy lot, the portion of thy measures from me, saith the Lord; because thou hast forgotten me, and trusted in falsehood.
The king and queen have habituated themselves by repetition to rebel against God. There is no indication in this text that the way they became habitual rebels was by birth as habitual rebels. God is asking a rhetorical question, "Can the Ethiopian change his skin, or a leopard his spots? Yes they can, but very few do so. The Jews knew about bleach and dye. Even so, very few Ethiopians do change, and very few leopards lose their spots; and it is highly unlikely that the king and queen will suddenly change their habit ingrained over years. It is possible, but unlikely, at this point. But if the occasional Ethiopian can and the occasional leopard can, so can the king and queen. "Then you MAY ALSO do good."