yeah, by modelling I mean 3d form with shadow and texture. The orange has the strongest shadow, it reads best in 3d, while the apple on the left has great observational detail and texture. The apple reads better as an apple, but the orange reads better as a 3D spherical object. What I'm suggesting is that on the drawings like the apple, mix in a bit of its complimentary colour to darken the shadow (dont use black for shadows!). In painting the general rule is to mix complementary colours for shadows (so if you're painting a red ball, you mix red with a bit of green to make the shadow for the ball), although I dont know how well that works with pastel.
SirGreensock- I see what you're saying. By modelling, you mean rendering in 3D in 2D space, right? In rendering, I'd thought that the apple on the left was most convincing. The orange seems symbolically rendered, with flat, obvious darks and lights. Please explain. Once again, thanks for consistent and strong critique.