My new study in Illustrator, In this design, I textured a flat style orange with grain textures.
I started by separating these shapes into layers of their own to make the texturing easy.
I duplicated the layer and then used the gradient tool to set a color range, I made sure I did this in the fill of the shape and then changed the blending type. I used the soft light and darken mostly. For the highlight and shadows respectively.