I am using a Raspberry Pi 3 Model B (non-plus) together with the official 7 inch touchscreen connected to the DSI connector. The latest Raspbian 2018-03-13 is installed. This works fine, but the display has not exactly the most vivid color rendition. I generated a ICC profile for the display with my Spyder 4 colorimeter using Argyll CMS and dispcalgui.
When I try to apply the profile using xcalib, nothing happens. The display output still looks the same.
The question xcalib not working on Raspberry Pi seems to be related, but it was resolved in a different way not compatible with my usecase.
Update: In the mean time, I also tried the different graphics driver options in raspi-config. With the OpenGL drivers, I get the error message
Error - unsupported ramp size 0
Update 2: When trying to change brightness with xrandr as proposed by @EricAnholt I get the message
xrandr: Gamma size is 0.
when using the legacy driver and no message when using the OpenGL driver. Both times, no change in brightness can be observed.