I'm not sure how well it would work, but you could try making a shapeburst gradient in each level. Photoshop's way of doing it is a little kludgy, though. Start by breaking each level into a separate layer. You'll want the lower layers to be filled in the middle because this technique is based on the edges of the layer's paint, and if there's a hole you'll get strange results.

For each layer, apply a stroke layer effect. Change it to inside and gradient, and in the gradient type, choose Shapeburst. This creates a gradient from the outer edge to the center. Click on the color swatch for the gradient, and set the right-hand point to the highest elevation color for the layer and the left hand point to the value you want for the edge of this layer (likely the original color of that layer). Then adjust the size until the gradient looks they way you want.

This will probably create some artifacts near the edges of each layer that you'll have to paint out by hand. The Healing Brush tool might be able to remove them automagically.

Once you have your gradients, you can do some additional painting to create more detail. The technique I prefer is to make a new layer, fill it with 50% gray, and set its blend mode to overlay. The layer will turn invisible, but if you use the burn and dodge tools on it, you can non-destructively raise and lower areas in your heightmap. The overlay blend mode makes any pixel that is at 50% gray transparent. Pixels that are brighter than that are screened over the underlying layers, and pixels that are darker are multiplied with the underlying layers.

You could also render clouds on an overlay layer, mask it with a completely black mask, and then paint in the random noise details by painting white or gray on the mask. Both of those techniques have the advantage that they won't change the original gradients themselves, allowing you to go back and make changes to any of the three layers without having to redo the work on the other two.

Finally, if you want some realistic-looking erosion patterns, you could take the heightmap through a free utility called Wilbur. It's on the technical side, but there are tutorials in its use here and at the author's homepage, and there's a very good chance that if you mention it by name the author, Waldronate, will show up to answer any questions.