You can do that as well. Using the two grayscale clouds layers create a new layer on top, hit ctrl+shift+alt+e...this merges all visible layers into the new empty layer. Then take that and image-adjustments-brightness/contrast and contrast to max and play with brightness, brighter = more lava and darker = more land based upon what you have so your land placement will not be random, it will be dependent upon the clouds. Then go back and do the color adjustments on the lava layers.