Sketching with Shades and Shadows
Architects are passionate about plans and elevations. Aside from thumbnail sketches, architects revert to plans and elevations to formulate their early conceptual ideas. A plan is an “organization-tool”, so to speak, it assists the architect in getting the client’s needs into a functional building package. An elevation on the other hand, is the start of the architect’s idea of that building. Usually an elevation is a 2D sketch which is sometimes difficult to visualize. By adding shades and shadows one can give a 3-dimensional effect to an elevation.
Sketchingjourney's Blog 
The darker shaded surface recedes while the lighter comes forward. That is true for black and white renderings. In a colour rendering, however, the brighter detailed surface will come forward. In this sketch, the surface above the roof is a contradiction to the rule. The rendering of the clouds and trees help to negate this contradiction.