I have already tried to paint an apple in the gouache. But I had failed to give it a round shape. Thanks to a lesson of the painting course of PTO, I understood how to make.

We must first understand how to work the shade on a sphere: article on the  round shape.
Then, it is necessary to know how to make a gradation in painting : article on the gradation.

Shade and gradation are really two things very important to know.
Here is what it gives ( oil painting ) :

a sphere of Sandy Taillan ©

Oil painting on  A4 format paper ( 21 x 29.7 cm : 8.27 inch x 11.69 inch) with gray graduation.

I still need to pratice.