Existing colour paint is not terribly old, and no issue with damp wall right?
Just paint over with matex for A4 paper white.

After ten years my matex still hold up perfectly.

Brand preference only lah, Matex do not dilute with more than 2-3% water *be very kiamsiap with the water for best results*.
KCC more solids but harder to stir.. it dries thicker
Matex very beginner friendly, easy to apply, my personal preference
Jotun.. somehow I find their economy paint not as cheap so I never use
Yes  otherwise cement suck up water like a sponge and you will definitely see the line there.
Easywash for colour paint.. else any matex also can for white. If it stick properly to the wall its quite easy to wipe down with soap and water.. I use economy white for kitchen wall and frying stuff the oil splatter everywhere so no point for me to use good paint.