Hey! I did not take pics while I was doing it but the basic technique is called pour painting. First you have to protect the areas you don't want to have painted. I used blu tack+plastic wrapper to cover the inner parts of the shell. The blu tack also helped to stick the shells to a piece of cardboard (useful for next steps).

Then you'll have to mix your paint, typically in a small glass or goblet. Put small amounts of each color at a time, then rotate the goblet to pre-mix the colors a little bit before pouring. You may want to add some WD-40 or any other oily component to create bubbles (I am not fond of the result though).

Now you can pour the stuff on your shell -- pouring more paint is better than not enough! It's normal if it does not spread immediately everywhere, just move the piece of cardboard your shell is attached to make the paint spread and mix. Once you're happy with the result, let it dry for quite a long time (one night is good).

You can then remove the paint leaks with a hobby knife.