You can simply touch it up with the original ceiling paint. Your options diminish quickly if this paint isn't available. If you know the brand, product line, color and sheen used on your ceiling you could buy a quart and try touching it up. There is a good chance that this will show as a spot on the ceiling. There is always a difference between new and old paint.

If the color is really close, try "feathering" the paint out over a larger area after completely covering up the wall paint. Do this with a mini hot dog roller. The idea is roll out away from the main spot allowing the paint to spread on thinner and thinner as you roll. This could blend it in. The main stipulation is the new paint has to be very close in its color and sheen.