Painting an aquarium ornament?

I bought a small castle for my 12 gallon desk aquarium. I really like the looks of it - with the exception of the yellow ground around the castle. The substrate in my aquarium is a dark black - a special soil that I bought to allow me to grow plants in this tank. I intend to plant it heavily and add a few fish, a couple of shrimp, etc.

I would like to paint the base of the castle - color the orange ground with a black color, to match the color of the substrate in the aquarium. Is there any way to safely do this? I had considered putting a very light coat of aquarium silicon on the base of the ornament and coating it with black soil, but I don't think that would last very long.

Thank you! I will look them up. Would aquarium silicon work also? I think I can find very small pebbles that are about the same color as the black soil, so glueing is an option. The pebbles will be more durable than the soil.