What is colour primaries?

In plain language

A colour space is defined by three coordinate pairs, one for each primary. Move those points and every colour derived from them moves too. BT.2020 and DCI-P3 differ exactly in where their primaries sit.

Why it matters in your setup

The display has to be told which primaries the incoming signal assumes, or it will interpret correct data incorrectly. That is why declaring DCI-P3 to a display expecting BT.2020 produces over-saturation and a red cast on skin tones — the numbers are right, the frame of reference is wrong.

For calibrated displays, custom primaries let you describe the panel's real capability rather than a standard approximation.

What the DR HDMI 8K does with it

In the DR HDMI 8K's Dolby Vision data block builder, primaries is a direct setting. Choose a standard set, or set CUSTOM and enter the coordinates yourself: red X and Y, green X and Y, blue X and Y, plus maximum luminance from 100 to 10000 nits and minimum from 0 to 1. Two of the built-in EDID tables are ready-made LLDV profiles, one DCI-P3 and one BT.2020.