The watercolour is going to be mixed into the silicone if I'm correct, before the silicone even sets. If you're really nervous about running you should be able to use acrylic instead, simply water down the acrylic and it will have the same drippy texture. I don't believe that the colour would run. I would be more afraid of the pigments themselves, you need to be careful about which you use as some are toxic. Also if you've never wired lighting before and that everything is safe and the snake can't touch it. DIY lighting sounds spooky to me IMHO but I don't know your background (although I DO know about paint).
EDIT: I have my snake's hide painted with (non-toxic) acrylic super duper bright hot pink and I've had no issues. The latex also sets itself and doesn't require further sealing so imho it's a good idea for animal stuff.
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