Painting ceiling tiles can be very tricky. They tend to absorb paint very easily so any spray lines or too much paint can ruin the tile.
Proper way to paint them is to make sure you are using a fine tip with a wide fan and that it is in good condition so that it is not fingering (leaving lines on the edges). A good tip would be something like a 413, 414 (fine tip) with low pressure. Our pot sprayers allow a lower pressure to be pumped out with a nice even finish.
You then simply want to mist the paint on. Plan on coating in at least two to three coats depending on darkness of color.
Some tiles will be more difficult to spray as some are designed to hang lower than the t-bar forcing you to not only spray the bottom of the tile but all sides as well. Simple spray lightly and cover all areas.
If forced to spray the tiles out, you still need to spray in the same fashion. A light mist with multiple coats of paint.
Danger of putting on too much paint. I have seen some tiles after they have been painted, bow so bad that they are about to come out. Some times, they will dry and go back flat but who wants to risk that. Too much paint applied too quickly will load the tile and force it to bow. Do not allow this to happen.
What type of paint to use: Flat Latex. Cloverdale Premium Classic Flat