Hi --
 
Yes, some paints are casein-based. If you do a simple google search for "casein + paint" you'll get some basic information.  Casein is used in other products as well, including soaps and lotions and glues.  Casein is nice and thick, you see, and before modern chemistry provided alternatives, it was a staple for quite a few crafts.
 
My web site contains a single paragraph and link about non-food uses of casein, but you may wish to just google around for further information.
 
This is, of course, not an issue if you're only sensitive to ingestion of casein.  And I think that it may not be an issue once the paint dries, but you should check further rather than trusting my guess.
 
--Beth Kevles
  Eating without Casein website
  web.mit.edu/kevles/www/nomilk.html
 
Disclaimer:  I am not a medical professional.  Please go to someone who is qualified when you need medical advice.
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