You could try "greenboard" that is more water resistant than standard drywall and is designed to be painted on. Greenboard will still rot if exposed to constant moisture.
There also isn't just one type of cement board. Certain brands are more easily finished with drywall mud and able to be painted than others. I just finished my bathroom using Denshield (which is a gypsum/cement mix with a vapour barrier) and it extended past where I tiled. The mudder simply blended it with the rest of the drywall and I painted over it. You can hardly tell the difference.
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