Hey pntr, granted your project was actually completed almost 2 months ago, but I'll say my peace, for your info.
Like many posts above, I almost always recommend folks remain or paint their ceilings white. Primary reason is because it 'tickles' the eye with the contrast you choose for your wall paint and it helps reflect light, thus ensuring your room is well lit.
But, if you insist that your ceiling is painted a color besides white, do this. Grab a professional paint color deck and match your wall paint to the swatch in the deck that most closely resembles it OR if you have the exact paint brand deck, even better. From there, look toward the very lightest value of that color. When you find that color, if it resembles an "off-white", you've found the solution to your question, but if it seems to be "heavier" than an off-white, go with the white.
As far as what sheen to go with, I'd have to disagree with a few posts above, I prefer either eggshell or satin, on the ceiling. If you apply flat ceiling white, to your ceiling, what do you plan to do the first time you find a stain on your ceiling? Doesn't matter what you plan to do because the only option that works is to clean the surface and repaint it. That's a waste of time in my opinion.
Best of Luck!