Umputun's tip works, but only as long as you turn off "Automatic graphics switching" every time that you reboot. (Thanks Umputun for the tip!)
It looks like an error with the gamma correction, which is how the graphics card displays your image - it doens't actually impact the stored image. That means that your screen capture will look normal to anyone who isn't having this problem. 😉
Basically, if you turn off "Automatic graphics switching" before you boot, the discrete graphics card will be correctly initialized on boot. Then you can turn "Automatic graphics switching" back on, and you won't have the problem until the next time you reboot. I've tested, and this workaround continues to work even after sleeping and awakening my computer.
For whatever reason, the integrated graphics card is always initialized with the correct gramma settings, but the discrete graphics card is only correctly initialized when "Automatic graphics switching" is turned off. I didn't have this problem when using Lion, so it's definitely an error with Mountain Lion. Hopefully 10.8.2 will fix this.