RayTracer, those are great! Here's a trick Cyb taught me: first, go to hue/sat and drop the saturation to 0, grayscaling each of your different layers. Then take one of these layers and go to image>adjustments>selective color, choose Neutrals and start playing around with the coloring until you like what you see. Play with the blacks and whites as well, until the layer has the coloring you want. Write down the different color levels, then go to each of the other grayscale layers and set their Selective Color to the same values. There may be a shortcut to this, such as selecting all the grayscale layers at once (using the shift key) and then going to Selective Color... But I haven't actually tried this.