The background is a contrast decision and nothing else

It gets discussed as a matter of taste. It is a measurement, and treating it as taste is why so many portraits fail.

What is actually being decided

Whether the outline of the head survives at the size the headshot is displayed. Dark hair against a dark wall produces a silhouette with no face in it — invisible at full screen, total in a list.

Why generic advice fails

"Use a white background" is wrong for about half of people. Light hair against white gives a floating, cut-out look; dark hair against white works well. Dark hair needs a lighter background, light hair needs a mid or dark one.

The rule is separation from your own hair colour, and that is a different colour for different people.

The second variable nobody sets

Distance. A metre or more between subject and wall throws the background slightly out of focus and removes texture that competes with the face.

For a group

The background must be identical across everyone, which means the per-person optimum is exactly what you do not want.