What rules are available to the architect who wishes to make such a rendering? There is the science of perspective, and that of shades and shadows.
It is usual to assume a conventional light source for a rendering. This, is at 45 degrees from above. However, it must be realized that this is only a convention, and that while a design might look well with such 45 degrees shadows cast upon it, as at noontime, the same design may not look well with shadows cast by an early morning or late afternoon sun at an angle of perhaps 20 degrees with the ground line. Therefore, it is well to study a design with several different light conditions before making a rendering.
Keeping these considerations in mind, observe the theories everyday on actual structures as you walk along the street. Notice in particular that the vagaries of light and the presence of atmosphere cause constant change in value and colour in every wall surface, under every projection. Become aware of the differences in value and colour of the many pieces of each building material, such as stone. Remember that no two things in nature are exactly alike. Notice that the texture and colour of different pieces of the same material are affected in different ways by age and time of day. Also, as the sun becomes bright or dull, the same building will take on a hundred different degrees of brightness- sometimes within the space of a minute. When you have absorbed these phenomena, you will be in a position to develop your own conventions for rendering light and shade under any condition.