How do I replicate colour assignments from an existing map in GIMP?

[Criv]

Hey,

so long story short, I'm trying to replicate the look of an existing set of maps. A fan of an RPG, Deadlands: Hell on Earth, did very nice evocative maps of about half the game area but has since stopped producing maps. I'm looking to fill in the regions that weren't covered by that set.

As it uses an alternate history US, I've tracked down the original topography maps that match the data in the existing maps. However, I'm having trouble figuring out how to duplicate the colour palette they used. I can use "Sample Colorize" to get a reasonable match of most of it, but the mountains don't come out looking right (there's a few other inexact bits, but nothing quite as awkward looking).

I've included a small sample for Colorado (one of the maps I have in the existing set). This includes the original data, a sample of the existing map, and what the best replication of the colour scheme I've achieved so far looks like.

Any help or suggestions would be greatly appreciated.