[tadfisher]

Are you speaking subjectively, or are you saying reducing the backlight level is affecting the color reproduction of your IPS display?

I've found that while I may need to recalibrate with my Spyder, color accuracy is essentially unchanged at 50% brightness.

[TimTheTinker]

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.