I'd be interested in doing this too it as I took A level sculpting classes at sixth form and loved it! The only comment I would make on using white clay is that you would have to make sure you fire it completely (or if it's air dry stuff then make sure it's solid). It could pollute the tank up quite bad.
As for colouring I don't have any clay stains myself because the only clay I use at home is super sculpey which is used for making miniature models and that can be bought in small packs off ebay. I think this would be best for aquarium use because you can make smaller, more detailed looking figures because of the give to it. I usually use acrylic paints to paint these but again, I'm not sure about these being tank-safe I'd be worried about the water being polluted with paint!
I'll get back to you if I can find any info on this.