I took a crack at what I think you are looking for - greens for low elevation, tans for high. See attached. I tested on the 4.00 version of Stalingrad '42 in the Scenario Editor.
You would replace your ...\Map\2DNormal...bmp files with the files in the attachments. There is a slight within-hex gradient, but, eh. My method also wiped out the subtle "texturing" within the hex. I probably could have figured out how to grayscale then gradient, but I was going quickly.
If you want to change the color range, I also included a blank template.
Use paint.net's gradient tool - Anti-alias = No, Blending Mode = Multiply. The top color is the Primary color, and the bottom color is the Secondary... higher elevations go towards the bottom color. Click in the center-top of the .bmp and drag all the way down to the center-bottom.
Once you're happy with the color range, re-size by 50% to get the "100.bmp" and then 50% again for the "50.bmp".