DO NOT use exterior paint. I know it seems counter-intuitive, but exterior paints are actually less moisture resistant than most interior paints since they (the exterior paints) are designed to be painted onto wood substrates that can expand and contract with the seasons.
The general rule of thumb for a moisture resistant and scrubbable paint is to use the hardest and least flexible paint that can be put on the substrate you will be using.
By "bps" I assume you mean Ball Pythons? In that case you don't need that much moisture resistance.
I would recommend sealing the floor with some sort of solid plastic, whether it be PVC sign board (aka Sintra), FRP, shower board, or some scrap vinyl flooring. Then the walls could be painted with pretty much any high quality acrylic latex paint.
A higher gloss finish will be more durable and easier to clean (and often slightly cheaper) but will show imperfections of the cage insides. A flatter finish will hide imperfections but not be as scrubbable or moisture resistant.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)