Hi guys,
First off, I have to mention that the software known as Wilbur is amazing, and even more amazing is that the guy who makes it gives it away for free...! I discovered it just a few hours ago and it solved all my niggly 'great-mountain-generation' problems; I had a height map ready to roll in PS, but the 'Render Lighting' method just wasn't doing it for me...

So anyway, here's what I've achieved using Wilbur plus the PS height-map:
Click image for larger version. 

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It looks waaay better than anything I managed in PS, mostly because the mountains now seem radically different to the lowlands (which is what I wanted). Now... up near the north I'd want it to be mostly white-ish coloured, since it's permanently snow, and down south I'd like a sandier, drier colour to dominate most of the southern islands, but I'm not sure how to easily apply this without ruining the Wilbur-colours for the mountains... i.e. I'd want the green to be replaced in both cases, but keeping the snow-caps, and definitely keeping the texturedness of the mountains and terrain.

I'm probably missing a pretty simple and obvious solution, but I am new at the whole cartography game (only started 48 hours ago, but it's been a blast )