Several factors. Black on white gives you maximum contrast.
A flat bottom matches the flat top of your front sight. And if you use a scope, aim at the corners of the black square. The cross hairs line up perfectly with the sides and corner of the aiming mark.
When shooting with irons, don't let the front sight touch the black. You will lose control of elevation, black on/in black. Leave a band of white between the sight and the target. A rifle/ammo is no better than the sights.
