Question

why the Solar Cooker is painted black from inside

Answer 1

black colour easily absorbs heat. to cook the food heat is required therefore they are painted black.

Answer 2

A solar cooker is a device which uses the energy of direct sunlight to heat, cook. The inner wall of solar cooker is painted black because 
the black colour is a good absorber of heat it absorbs heat and makes the temperature hot inside the cooker.