I think it's the Gamma control. Snow Leopard changed the default to 2.2, thus it look different from previous OS versions. I don't know if W7 is the same or not.
As for W7, it's the color scheme that makes it look like it's more brighter and colorful. If you were comparing it to Vista, you would say Vista was just dark, dull and boring, due to the color theme used.
Snow Leopard is starting to get dull and boring for me, the grayscale|Aqua blue is just old now.
Because you are either using different monitors, or different color profiles on the same monitor. Windows and Snow Leo use the same default gamma (2.2) so they should look the same given the same hardware and profile.