On a Mac Pro with Thunderbolt displays running the latest beta of El Capitan, or even a Retina iMac, System Profiler reports the color depth for the monitors as CGSThirtyBitColor, as opposed to the usual 32-Bit Color (ARGB8888). More so, on betas 1 and 2, this string was correctly localized as 30-Bit Color (ARGB2101010). However, I've been unable to verify that anything going to these displays is actually 30-bit color.
A visual check of 10-bit media in QuickTime shows that the 10-bit data is being quantized to 8-bit (as it did in 10.10 and earlier), so I assume this needs to be properly configured at the application level if it works at all. NSScreen still reports 24-bit color.
Could this be a capability that can only be achieved in an OpenGL/Metal context?
For some more info on this topic, please check out the OS X El Capitan 10.11.2 release notes: