For what it's worth, here is the technique that I used on my 1957 Michigan
After working the dents, etc out, I primed and painted the plate with the base coat of color and let it thoroughly dry. I covered the entire plate with a good grade of masking tape which had really good adhesive. You MUST be very thorough about being sure that the tape is stuck to the contours of the sides of the characters, symbols lines, etc. on the plate. This is a very important step.
Then, using a vibrating type of sander held flat, carefully sand the flat surface of the characters to remove the excess tape and expose the surface to be painted with the contrasting color. Prime and paint the numbers. After they are dry, carefully remove the masking tape
Lastly, cover the entire plate with a clear coat, to protect any slight imperfections in the seam at the edges of the numbers and the background color.
I used this process on the plate with excellent results about 4 years ago, and it still looks new.
Lake Orion, Michigan