Depends on the format, but 10 or 12 bits per color primary is typical; moreover, HDR video standards typically use limited-range YCbCr encodings instead of full-range RGB, so certain low and high values are defined as "blacker than black", "whiter than white", or are otherwise reserved.

So in terms of hex digits, three sets (Y, Cb, Cr) of three, with not all values representing valid colors.

Compared to SDR standards like sRGB, HDR formats also typically use larger color spaces (Rec. 2020[1] is typical) and far more extreme transfer ("gamma") functions (PQ[2] or HLG[3]).

Finally, note that it is common for the encoded values to represent colors and intensities that far exceed the capabilities of most, if not all, display hardware, so the mapping from encoded values to actual displayed pixels can be rather complicated. Google "HDR tone mapping" for more than you ever wanted to know.

[1] https://en.wikipedia.org/wiki/Rec._2020

[2] https://en.wikipedia.org/wiki/Perceptual_quantizer

[3] https://en.wikipedia.org/wiki/Hybrid_log–gamma

HDR is less about the colors and more about achieving a wider range of brightness levels (hence increasing the total dynamic range). It's typically encoded using 10 or 12-bit color depth, and metadata is added to the media on how to map the colors' luminance/brightness values (e.g., what darkest and brightest values of a image/video should be). This is then used to transform HDR content into appropriate color and luminance values for your specific monitor (e.g., the reference monitor used for grading might have a peak brightness of 1000 nits, but yours might be different, or have a different luminance-response curve or support a different color space).

In SDR content, colors are encoded with 8-bits ranges. So, 24bits for the three color channels. With HDR, we usually use 10 bits instead (this is where the "10" comes from in "HDR10"). See https://en.wikipedia.org/wiki/Color_depth#Deep_color_(30-bit....

Both QR codes are using #ffffff.

The "bright" one has metadata that tells the operating system to render white at the maximum possible brightness, instead of whatever brightness it would normally render white at.

It's broadly supported on Apple devices, though how well it works depends on the hardware you have.