Originally Posted by Richard Paul
It would do much more than that since it would shift any of the colors that include green (which has a different color coordinate in the Adobe RGB color space). Even with a color space that only uses monochromatic primary colors you still have to pick the frequency of a color and for example there is a very noticeable difference between 610nm red and 630nm red. Both are red but they are not the same frequency of red. For displays to maintain color accuracy the primary colors must be mathmatically defined.
The output device knows that it is outputting BT.2020 color, or whatever other colorspace it needs to, and adjusts the values it is sending to the display accordingly.
100% red, 50% green, 20% blue is a different color depending on the colorspace it's assigned to.
In the case of computer calibration, measurements are taken of the display with a meter, and a custom profile is created based on those measurements, so the computer knows exactly how much of each color component to send to the display, to display accurate color.
Any values a player supporting BT.2020 will output, will be specific to the BT.2020 space, rather than being custom tailored to your display, just like any player today outputs values assuming they are being displayed in the BT.709 colorspace, and applies the transformations required to convert BT.601 to BT.709 in the case of upscaling DVD players.
If you play a Blu-ray (BT.709) on an upscaling 4K player which is outputting BT.2020 color to the display, it would for example transform "100% red in BT.709" to a value of 50% red - assuming that BT.709 red is only 50% as saturated as BT.2020 red; I don't know the real value off the top of my head.