Old paint on ceiling.. best way to hide it???

The previous owners of my home were quite sloppy when they painted! I have white popcorn ceilings and in my childs room, the lilac wall color is on the ceiling portion. I dont want to paint the entire ceiling, but is there a product I can use to hide it? Would my latex primer work?? The hard part is not knowing what is on there now and I would not want to touch it up and it be noticeable.