Colour balance problem

I think I have done something daft (no surprise there) but, when I am using Aperture ('modern' imac) and send a file to be edited in Photoshop, the colour balance changes drastically. Both pictures, side by side, are disturbingly different. When the file is saved, it goes back to the original colour balance.
This is a new problem and I have probably been tinkering where I shouldn't.
Has anyone any idea what I may have done and which particular settings I should be looking at?

Perhaps the best place to start is by looking at the "Colourspace" setting.
Just as a guess:
Aperture is set to sRGB
Photoshop is set to AdobeRGB

The two settings will show a different range of colours.

Then, can we assume that you have two different monitors - one for the Mac and one for a PC. Or, are you using Photoshop on the Mac as well? If so, the different RGB settings are probably the cause.
Geoff

Great stuff - thanks Dr. J.
I had put Aperture onto 'onscreen proofing', which does its best to show you what the print will look like. Without that switched on, the pictures are identical.
RESULT.