Recently, my stepfather and I fixed a portion of my ceiling that needed replaced. However, after he, the handyman, was gone, it was left to me to paint the ceiling, including the space where he had used to even it out with drywall joint compound. I painted the ceiling and it looked pretty good, but parts of the joint compounded area didn't look good enough in my opinion. So, I decided to prime the spot and paint over it. It looked worse, so the genuis that I am thought one more layer of primer and paint would make things better, Needless to say, I now have a spot on my ceiling that looks almost gray rather than white. What do I do? I tried sanding it, but it is hardly making a dent in it.
It sounds like the dark spot is either dip or a hump and that is what is making it look at different color. You need to skim it out with more drywall mud to get it even and then prime the entire ceiling with a latex drywall primer and finish it with a flat latex CEILING paint. Don't use a regular wall paint as it will have to much sheen and will show up the defects in ceiling.