Crystal buttons on a strong colour do something they cannot do on a pale one, and where they are placed follows from that.
On a light fabric a crystal reads as a small bright detail among other details. On a saturated blue it reads as a point of light on a dark field, and the eye connects those points into lines - which means the buttoning grid stops being a texture and becomes a drawn pattern.
So on a chair like this our upholsterers check the grid from a distance before fixing anything, because a diamond pattern that is a fraction irregular is invisible on a pale cloth and glaringly obvious on a dark one. The buttons are positioned on the frame, checked at three metres, adjusted, and only then pulled.
They are also pulled slightly shallower here than on a pale fabric, because a crystal that sinks into a dark ground disappears entirely - on white it would still be visible as a dimple, but on deep blue the shadow and the fabric are the same colour, and the light is all you have.