Hmmm... I bet you are in CMYK mode. If so, convert to RGB, then flatten. But, even then you may end up with some changes in the conversion process - the only way to really avoid that is to start in RGB.
(edit: ehh... never mind... I may be way off with that now that I've tested it a bit.)
If that fails, you could make a new layer at the very top of the layer stack, and then make a merged copy (Ctrl-Alt-Shift-E for windows). That might get you a good representation.
(Those are the shortcuts, this is a very large topic. If they don't work, try working your way through this:
http://www.normankoren.com/color_management_2.html . Look especially at the section on conversion options. You may also wish to check out the book "Real World Photoshop" that has some good info on color management.)