Ronnie, I try to find a picture (or a bunch) and I model it in 3d using 3d software. There are different techniques to do that.
In this case I used polygonal modeling. That is moving points and simple shapes in virtual 3d to create the model.
There is also digital sculpting. You start with a primitive shape and give it enough geometry that you can move the surface in different ways just like clay. Best way to get organic look in my opinion. It can be a lot of fun.
There is also curve modeling but I don't use this method so not familiar with it.
The question on the color of the final render is the based on the software/method they used to render their depth map. The picture is based on the depth parameter from the camera in 3d space. The render uses different software and techniques (math algorithms) to calculate the bright and dark portions of the image. The cnc software converts the different colors to grayscale first before converting to g-code so it really doesn't matter what the color starts as, as long as the converted grayscale is properly associated with the correct height. Scale 0 to1 equating floor of cnc to highest portion of the pattern.
I can make my depth maps any color I want but that is an additional step for my software that is not needed.
There are also other ways to get geometry into virtual 3d without modeling: Laser 3d mapping, photogrammetry, voxel clouds, and probably a bunch more.
Sorry for all the boring stuff.
I have this old truck model I've shared before so you may already have it. Here are some of my other patterns I have shared previously with the community.
https://www.pinterest.com/oscarluis32/gray-scale-depth-map-images/