I have found a serious issue with video playback using the RX480 and Crimson drivers, I believe now after measuring the output against an Intel IGPU that this is because AMD have omitted essential advanced settings for video (that would set correct 16-235 colorspace output for video playback) in the Crimson drivers.
I am a semi-pro display calibrator, calibrating my own displays for over 10 years and I have found serious greyscale and Gamma issues with the Crimson drivers, see results below.
The older Catalyst drivers used with older pre-RX400 series cards had the options to setup video playback for the correct colorspace for video levels 16-235 along with the global GPU pixel format, the correct settings to achieve bit perfect video output via HDMI was to set the AMD global pixel format setting for the GPU to 0-255, then under video settings (removed in the Crimson drivers), Dynamic range was set to 16-235 (there was also a registry hack tool to set the GPU to 0-255 pixel format, this no longer works with Crimson), all other enhancement settings - disabled.
This is not possible with Crimson drivers, all we have now is the global pixel format setting, which allows a setting choice of 4:4:4/4:2:2 YCbCr, 4:4:4 Limited RGB 16-235 and 4:4:4 PC standard (Full RGB).
I have calibration equipment and software and have measured all the available Crimson options for Pixel Format and the only option that provides a stable bit perfect output is 4:4:4 PC standard (Full RGB), unfortunately this is unusable with a 16-235 TV display and becomes heavily crushed due to the incorrect colorspace (Dynamic range setting (16-235) fixed this to the correct colorspace in previous CCC Drivers).
The available 16-235 Pixel format options - 4:4:4/4:2:2 and Limited RGB settings clip greyscale - black AND white levels and the upper gamma is therefore distorted due to this clipping, currently these drivers are unusable for correct video and bluray playback with degraded image quality on a 16-235 video display, measured quality is broken and distorted, I have now disabled the RX480 until the Crimson driver is sorted out, (if not I will have to take the RX back as it is currently no use for me like this and get a 1080 Nvidia, shame as I have always used AMD) and reverted back to my onboard Intel HDMI output which there are no video issues with both viewed and measured results, calibrated results below.
Please AMD return the advance video settings that have been omitted in the crimson drivers, which included the important Video setting - DYNAMIC RANGE.
These are my test results/attached below of the Gamma/RGB output via HDMI from an Intel 530 onboard GPU (I7 6700) against the AMD RX480 8GB, using the current Crimson drivers ( I have also tested all Crimson drivers from first release and they are ALL the same results).
Intel 530 HDMI 4:4:4 Measured Output Results
AMD RX480 4:4:4 Measured Output Results