Color temperature switching on a MacBook Pro (2011) while switching graphics engine

I recently ran into the problem that my MacBook Pro (early 2011) dramatically changed the color temperature on the LCD while it changes the graphics engine from “integrated” to “discrete” (I use gfxCardStatus to make this visible).
SMC reset, NVRAM/PRAM reset (see Apple for details) did not help at all, although “Google” reported that this should help.

What actually did the trick: I had plugged in another monitor using a Thunderbolt/Displayport to DVI adapter and I assume that I switched off the MacBook in an obscure way. So I just replugged this monitor (interesting fact: It did only work again after a Windows ™ like reboot), rebooted (see before) and unplugged it again.
Now everything is working as it should …

Things you shouldn’t expect working with a Mac!