Here’s what I’m currently working on:
There are 6 more layers left to go. This first layer was done by 3D carving a heightmap gradient into the substrate. This one varies from about 2.0 mm deep at the bottom to a little less than 1.0 mm at the top over a vertical height of 70 mm. It was subsequently filled with colored epoxy resin.
Since your sign is likely larger you could use an even steeper gradient if you wish to get even more contrast. If you use a substrate other than white, spray the recesses with white lacquer before the epoxy. Sand that paint off the surface later. You will have to experiment with the proper density of colorant.