- Make sure you have Antialiasing set to at least 2x in Quality Settings
- On a Camera in a scene, check the HDR option
- Click on the Background color field to open the Color Picker
- Try adjusting the values of R, G, B using the sliders
Console gets spammed with:
"“HDR and MultisampleAntiAliasing (in Forward Rendering Path) is not supported. This camera will render without HDR buffers. Disable Antialiasing in the Quality settings if you want to use HDR.
UnityEditor.HostView:OnGUI()"
and the Editor feels very sluggish

The warning exists in 5.0 as well, but the Editor doesn't seem to move so slow (at least when making changes in the Color Picker window)