Yeah, I tend to just play around with my values and add shading and coloring to try and give cumulus clouds a lot of fluffy mass. The other thin type are really easy though, lol. Putting a little texture on them helps quite a bit as well, im my experience.
I have a lovely cloud brush I made that I used for this pic. I would be so lost without it.
And yup, there are!~ You should check out dA. I've used my fair share of them when I was starting out in PS before I figured out I COULD paint them if I could find a good tutorial (which I eventually did).
Link
WilkFiadh
Nice work on the shading. Though, am I the only one that thinks clouds are mega fun to paint?