I have recently done the same thing. What I would do is prime your plug with an epoxy primer such as Klass Koat or automotive primer such as Transtar. Next would be a coat of color, again either Klass Koat or any catalyzed single stage paint. Pull your mold from the plug by prepping with wax and PVA. Trying to find other systems that may not contaminate your plug may result in not being able to remove the plug from the mold. When it's time to clean the plug, wash several times with a scotch brite pad and Dawn dish soap, acetone wash changing rags frequently then wet sand off the color using a solution of Dawn and water. Once down to primer acetone wash again and then spray on a light coat of primer.