When they made enamel in India they left the ground to be enameled unpolished, so the light would reflect better through the different colors of glass.Have a look at the red part.

• 当他们在印度制造瓷釉的时候；他们对于底层不涂以瓷釉；因为这样就可以反映出更好的不同的颜色.；看一下红色的部分