Light

In all digital painting and concepting, light is one of the most, if not, the most important factor in a readable and attractive art piece, as it's the way we perceive all objects. Painting light can also be a very difficult simply because light is something that everyone recognises quickly, and while not everybody understands the laws of depicting light artistically, it's very obvious when light appears wrong. Let me describe the basics of light behaviour.

As you can see from the diagram, there are various components in shading an object, and there is often more than one light source. Lights are rarely perfect white lights, and shadows are rarely totally black. This is how an object usually appears outdoors on a normal, sunny day. There is one main, small and bright light source, which is the sun. Naturally, this casts shadows in the same direction that the light is shining. But one thing that often is ignored is that there is a massive secondary light source outdoors, which is the sky. In this example, the earth's atmosphere has been hit by sunlight, and the nitrogen/oxygen atmosphere has scattered photons at  the blue end of the light spectrum. This gives a soft and bluish secondary light source. The cast shadow of the object is frequently coloured by the light of this secondary light source, combines with reflecting light from the primary light source. This bounced light will also often illuminate the side of the object closest to the cast shadow. In this example, the terminator is the shadow of the object least illuminated by any light source, which still often is coloured by some bounced light. Hence, to depict realistic looking objects in your concept art, you must seriously consider the direction the light is shining from, and also the colour of your lights and shadows. This will greatly enhance realism and believability.