How do I do a realistic color change on a realistic fractal map ?

[Deadshade]

My skills aren't that great, but for what it's worth, I use Selection / Feather tool a lot when I want to blend. It can't be used in every scenario, but works great when you want to remove the edge from a block of colour.

I am doing that a lot in Wilbur to avoid too sharp cliffs too. However the problem is that I don't really want just to blur the edges (this then looks like a smudge separating 2 textured areas) ) but rather create a transition where the proportion of pixels of a given color gradually changes from 100% to 0% when one goes across.
The reason I so intensely look for a solution is because this is the only way to do a very realistic map. In the nature when one goes from a jungle to the desert, the textures and colors gradually change - there are no edges, no blurs and no sharp boundaries (well to be accurate, the sharpness depends on the scale and the nature of the biomes - ice/tundra is rather sharp but jungle/Savannah isn't.)

So this is a very general problem as soon as one wants to do a very realistic map on continental scales - the shapes/textures and colors for a biome are given but then one has to manage a believable and gradual color and texture transition from one to the other.