painting the outside is fine but if you have to paint over silicone it can absorb some of the toxins from paint and leech into the tank water, that is if you paint while water is in the tank. but if you paint over silicone and let it completely cure, while no water is in tank, it should be fine. i have done this to one of my tanks, that i sold to a friend, with no adverse reations and i also plan on doing so again to my new tank, which will hopefully inhabit a future jar.
thanks for the correction!