Graphic Converter vs. Preview

not sure but i've noticed the same thing. i can say that Graphic Converter is by far the more powerful app. it amazes me sometimes with what it is capable of, as well as it's very deep support for AppleScript. i know Preview has been optimized quite a bit in recent versions. it's incredible how fast it can read large PDFs now. perhaps this optimization also included some scraping of the code that reads and displays image data. what might be an interesting test would be to open the same picture in both apps. then export to the same format from each app, jpeg for example. then open both of these in the same app and see if one looks better than the other. as long as it doesn't trim the quality on export then i could deal with the lower quality previews in exchange for the speed boost. but if GC indeed exports better quality images then i'd definitely go with GC for all my exporting needs. wish i would have thought of doing this sooner! 🙂

As with Photoshop some applications use a variety of embedded "profiles" to define what color space they are being viewed in. I'm not sure about either of the 2 programs you use, but maybe searching through the menus to see if there are preferences you can manage could help. Second would be to print out the image to a quality printer and see which one most matches what you see on the monitor. Its quite impossible to get an exact "WYSIWYG" (what you see is what you get) on the monitor seeing as how it uses RGB in a display mode (subtractive color) and any printer weather CMYK or RGB has to print in additive colors.
RGB can only be simulated using either Cyan, Magenta, Yellow, and Black, or with a 6 color printer greens and oranges are used.
Another thing to do is make sure your monitor in calibrated to your wroking environment. There are basic calibration that I belive comes with OSX or if need be you could spend $$$$$$$ on professional calibrating hardware and software.
For example: http://www.gretagmacbeth.com/Source/Gm.asp