This is a pretty long topic. One of the last few issues of FSM had a good article on weathering, which is what you are trying to do with shading. Essentially, their is a pretty strong school of thought that says that you need to "lighten" a particular colour in order to accurately capture the "scale" effect. In other words, as you make a model smaller, the original colour used on the full scale version needs to be lightened to accurately reflect (no pun here) the colour on a smaller version of same. What many modelers do is spray a darker colour (eg black before dunkelgelb or the common dark yellow seen on german armour) over all the joints (where metal plates meet on the real version) and on rivets, bolts, etc. Then when you spray the main colour on the model, these areas will be slightly darker to simulate what happens on the real deal with shadows from light etc. What you can also do at that point is spray the inside of these panels with a lightened version of your base coat and this will further accentuate this effect. There are some great books which explain this in detail with step by step photos. When you combine these techniques with washes and drybrushing and pastel weathering, you get a really amaziningly realistic effect. I don't have the names of these books here with me but hopefully someone else will provide some examples. Even try a google search for weathering models or use the search function on this site to get more information. Hope this helps some.