You can paint latex flat over latex flat without problem.
If you have a satin sheen on there now, it is critical to know if it is oil based or latex. If oil based, you need to use oil enamel over it, or prime with an oil primer before using latex.
If you use a deep color, you are going to have problems; probably take 3-5 coats. Otherwise, color is your choice.
I recommend using latex flat on walls and ceilings because it does not reflect and thereby hides imperfections.
For more on painting, read that topic on my website at www.bobf.info.