Then new wording from the AGM 2019 is: The surface of the covering material on a side of the blade, or of a side of the blade if it is leftuncovered, shall be black on one side,and of a colour clearly distinguishable from black and from the colour of the ball on the other.
This should mean that you may paint the uncovered side black, and then use another (legal) colour on the other side. Black is not defined in the rules, so "any black" is ok.
Depending on the new T4, it might be more difficult to get away with painting the uncovered side yellow or pink, and then use a black covering on the other side. There will have to be some rules for that.... If there is a definition of yellow (or other colours) in the new T4, you would probably have to paint the uncovered side according to this definition. Then there would have to be a test...