I'm a little bit confused with the transition from just setting solid colors to object to involving light and its different components. For instance, the following formula $R = I K_d max(0, n.l)$ that computes the involvement of the diffuse component of light in the object's color. The part where we compute the dot product makes sense because indeed visually points in the object where the normal is orthogonal on the light vector should be dark. However, I can't get to understand what's the purpose of $K_d$. Is there any example of an object in real life that would for instance have low $K_d$ (hardly react to the existence of a light source)? Why would changing $K_d$'s value ever make sense?