....On the "theory" of Color Modulation, it is NOT only for depicting the angle of the sun ( a single light source ). This is one way to do it, however there a quite a few others.
An object can reflect light from more than one light source at a time ( Light from the sun) , light reflecting from below for snow, and street lamps, reflecting from other vehicles etc.....the possibilities and variables are endless.
Also worth noting is that if an object is recieving light from more than one light source, each of these light sources are likely not all the same intensity. If this is reflected on the model, the possibilities are endless.
There is also another way to look at Color Modulation and this is perhaps the most interesting. That is to simply create contrast between distinguishing features of a vehicle or object. In this way we are simply drawing the viewers eye to the characteristics of the vehicle that we find most interesting or defining of the vehicles sillouette. When this thought process is employed, the Color Modulation may not ( likely will not ) follow the predominant light source. However, the result will look amazing! This would simply be a matter of interpretation. Each of us interprets visual stimulus in a different way....all based on our environment and past experience.
Is Color Modulation always realistic???? NNNOOOOO! Neither is assembling & painting little pieces of plastic and claiming that they are "realistic" as if they are a more credible representation of a massive hunk of steel. It is an artistic allusion and my hat is off to those who push it as hard as possible to create a piece of "eye candy". I have looked at many "perfect" models that were perfectly boring. On the other hand, I have looked at some that are a little bit less "realistic" but they were interesting to look at and could be looked at for an hour! I suppose it is all a matter of taste and there is NOT a right -or- wrong at all. If you enjoy creating or viewing a specific style...then it is correct for you!
Color Modulation is only new to modeling.....it is not a new concept to art in general. We must also keep in mind that all new ideas are treated with skepticism at first. I am sure that when Francois Verlinden started using "DRY BRUSH" to shift colors on a model's surface, some people thought it was crazy....then it became the norm.....then it became "old school". All techniques can be looked at from this perspective. The key is to take from each new technique and use what you like. Constantly growing as a modeler and keeping it fresh.....and most importantly make yourself happy with your models. If you like to paint your Sherman "OD GREEN" from a Tester's can of spray paint....by all means do it! If you like to play with shifting tones....please do it! If you like to paint your Sherman Pink with yellow Polka Dots....please do and keep up the good work!