[jasomill]

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