The displayed center of mass is not an entity or a constraint; it's not a part of the sketch at all. The color is the same as for the origin of the plane shown when you export in 2d, since they serve similar functions.

The displayed center of mass is not an entity or a constraint

OK, but I suggest it should be "entity".

May I request it here? It would be really useful for export drawings where "Center of Mass" could be shown.

It can't be an entity; given how SolveSpace currently works, entities (things you can constrain / constrain to) can't really be based on solid geometry.

On the other hand, exporting it seems reasonable.

It can't be an entity; given how SolveSpace currently works, entities (things you can constrain / constrain to) can't really be based on solid geometry.

Why not?

Suggest center of mass entity is similar to area ("show are of closed contours") entity, but instead of area entity which position is constrained to center of mass of closed contour in 2D, center of mass entity should be in 3D (similar to comment entities which already could be placed in 3D, but center of mass entity should not be dragged by mouse and its origin position should be always constrained to calculated center of mass position).

Why not?

Because generating a sketch is a two-step process (first it is solved, second the solid geometry is generated based on the solution) and it would be difficult to change that. I'm also not really interested in any abstract discussion of features with you specifically, because you consistently fail to engage in such discussions in good faith.