make a new layer above everything, or your bg layer, or whatever you want to color
set the blend mode of that layer to overlay and drop the opacity of that layer to 50%
get a cloudy brush, like the ones from doom abstract set 3 (can be found at www.deviantart.com)
make your foreground color a color you want to use (ie. red, yellow, blue)
brush in the areas you want to that color, then switch your foreground color to something close to what you used before (ie. if you used blue, try green or purple, if you used yellow, try red and orange)
after that you can either use another color/s or just make a new layer again
set that layer to overlay as well and drop the opacity to 50%
start brushing around with the colors again
(Tip: if you want to make some parts "glow" or give a ambient look to em, to lighten them up. Try using a soft yellow as your foreground color when brushing on top of red or orange. Or a soft purple on top of blue or green. This must only be done on your second layer of coloring tho)
Added Step:
If you want to, you can make a color balance layer on top of your coloring layers as well, just dont over-adjust the sliders... keep it at 30 or below...
after making the color balance layer, go to layer>remove layer mask>remove
then go to layer>add layer mask> hide all
then go to filter>render>clouds
will give the coloring another nice touch
