Glenn's idea is a great one--plus has the advantage of being very, very easy. Don't forget you could also go very low tech and draw with another color brush (perhaps set to "color"?) to break up the color monotony as well.

On a side note, I would also recommend lowering the saturation of the entire image a bit; right now this is very neon--which in fact may contribute to the pattern showing up, too.

A nice map.