Display brightness establishes the absolute limits of its output color space (i.e. the display's dynamic range).
By which I mean: bright colors won't be anywhere as bright with a dim display as they would be when the brightness is turned up. And the difference between dark and light colors is more pronounced with a higher brightness level.
I've found that while I may need to recalibrate with my Spyder, color accuracy is essentially unchanged at 50% brightness.