if the tiles have become yellowish and stained with water or whatever, you will have a slight obstacle in covering this even with a good primer. But it can be done. I would first consider replacing any tiles that are chipped, broken or damaged. the outer skin seems to accept paint well, but when gone, it does not.
You will need a spray rig, possibly a rental, for a better job and one that will apply a smooth coating. I would apply how many every coats of a good latex primer on the surface until the stains or yellowish had dissappeared.
Then spray on a good quality latex paint on the primer one from a paint dealer store.
Spraying, I would be careful not to have the spray gun, if this is your route, too close to the work as the pressure will blow the tile up and out of place, releasing all sorts of dust on your work. If the area is not too big, you could buy one of those cheaper waggner spray painters for about $70 and do the work. If you do this, you will need that goose neck extender tool so that you can hold the canister level and bend the goose neck up to spray the ceiling.