I’m running an NEC wide-gamut display alongside the iMac 27″, and while the iMac was being used only as a secondary display, I’ve been scratching my head for quite some time that I couldn’t get colours in Photoshop CS5 to match across both monitors. The colour-critical display showed fine, but when images were moved over to the iMac (usually for reference), the colours displayed much more saturated.
All other applications were fine, including Lightroom, so it didn’t impact on workflow too much. However, it started to drive me insane when I sat down for a lengthy printing session, so it was time to investigate.
It turns out that with the introduction of OpenGL rendering in Photoshop, if you run dual screens your secondary display (i.e. the one without the menu bar on the Mac) is effectively NON-colour-managed. The only solution I discovered is to turn OFF “Enable OpenGL Drawing” in the Photoshop Performance preferences.
My concern is that if you are running two displays that are similar in gamut, you may be being bitten by this bug without realising it, and making incorrect colour decisions.