The numbers are etched into the dial. There are various ways to infill recesses like that. Here I use model enamel paint and overfill. Let dry then come back with naphtha on a wipe to lightly wipe away the over-filled stuff. In this case the black coating will not react to the enamel or naphtha. Not always perfect- you can see the right line of the four lines gets skinnier for the upper 2/3 from the wipe pulling a bit too much paint out.