dab some tremclad or model paint over the numbers then wipe it off with a rag just don't get it in the knurling
Here is another technique that may work for you.
Use a common crayon of the color you want for the lettering. "Color" it into the numbers so that they are 'full' to the top, then hit the numbers with a flame from a lighter or match to melt the crayon so it fills and levels. After it cools, buff with a soft cloth to remove any overage.
I couldn't believe how easy it was when I finally tried it on my Black Beauty. Made the numbers look brand new.
Ben
I am a pro at doing this now. My method. Black testors paint, shake it up, grab a qtip dab it in the paint I mean really dab it good, lay the qtip on the number, what you have now is a large dot of paint on the number so much you should not be able to see the number. Wait 10 seconds then with yor index finger swipe the drop of paint away. After the swipe dip a new qtip in nail polish remover a light dip then while the paint is wet remove the light film of paint there should not be much. You do not have to wait for the paint to dry before moving to the next number. I posted a video in discussions showing this method.