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