A short answer, yes you can paint cement board. Will it last is the question. Cement board is very porous and will take paint easily, will the paint stay on the surface???? Can you explain what project you have in mind for the cement board???? That way you will get a better answer to your question
After thinking about my answer it is geared towards durarock. There are cement boards that simulate siding for the exterior of a home.
What is the substrate that you are asking about????
Fantasy Walls and Windows