Wow, that's gorgeous!
For borders, you might try either dotted or dashed lines, in a brown color that'll compliment the existing colors. That way you have something well-defined enough to stand out without clashing. For the labels, maybe do them in the same brown as the borders, but in a lighter opacity so the terrain bleeds through a tiny bit?
And good to see you posting again, especially if this turns into an AH map. I loves me some AH.