Texture mapping can be used to alter some or all of the constants in the illumination equation. We can simply use the texture as the final color for the pixel, or we can just use it as diffuse color, or we can use the texture to alter the normal, or... the possibilities are endless! Phong's Illumination Model

Constant Diffuse Color	Diffuse Texture Color	Texture used as Label	Texture used as Diffuse Color