Good question, and confusing one at that searching around for info on the subject. Unless things have changed with the release of Titan X and 980Ti, supported color depth seems to be an issue of what is driver locked on consumer cards (Titan X and below) vs what is natively supported (10 bits per channel on up) on professional cards (Quadro series) in Windows. The options for higher than 8bpc may not mean that applying them will work as they should without all hardware/software supporting it. As for your monitor, " 1 Billion" color support should be a  10-bit 1.07 billion color palette, but best to make absolutely sure your panel is a 10-bit one.
 
Perhaps someone a little more tech savy or EVGAtech can shed some light on this.      

Yes, with 10-bit color you get 1,024 shades of each primary color & over a billion possible colors. 
 
10-bit color has been supported since the 200 series... http://nvidia.custhelp.co...on-nvidia-geforce-gpus

My monitor is a Samsung U28D590D. I'm running Windows 10 Home 64-bit. And Nvidia driver 358.50
In Display/ Change resolution there's a setting for Output color depth that has the choice of 8 bpc or 10 bpc

I'm using DisplayPort 1.2. The monitor doesn't have a HDMI 2.0 connection, only 1.4
Seems this question answers itself. Obviously the card supports 10-bit since it's there on the driver menu. The DP connection must handle it as well since they wouldn't make a monitor a billion colors if no cable was capable of providing the signal. The Nvida Control panel legend does state that the program you're running needs to support 10-bit and probably some don't.