Here's the process I followed when working on the cover art for the latest issue for Worldbuilding magazine, V3I6 - "Economics".
I was working on this by colouring in the values in grey-scale, overlayed by a gradient scale and a couple of colour layers. I'm definitely looking to improve on this process and any criticism/advice on how to improve would be greatly appreciated. You can also go check out the issue itself here: https://www.worldbuildingmagazine.com/2019/10/economics