Panel Shading deals with the discoloration that is common in specific areas of a given section.... typically, I start off with a darker shade and do the whole tank, then come back with the SAME color, blended with some tan or beige (not white!), and starting from the center of a section, I lightly overcoat and move the brush spray towards the outside. This will give you darker edges, and a lighter center.
A particular tip - not all panels would "wear" the same... some get replaced, others get more exposure to sun and weather (such as the deck and glacis).... so try to eliminate a "pattern" look to your work.
I suggest you first experiment with some sections of white posterboard... start by spraying a dark green square (1 inch x 1 inch), and then practice the center-out technique until you are happy with the results.
You can find a lot of photos in the book by Jerry Scutts called "Basic Military Vehicle Modelling" (Compendium Modeling Manual #3 - ISBN: 1-902579-07-0)
And here is the link on Amazon.com
http://www.amazon.com/exec/obidos/tg/detail/-/1902579070/qid=1065681303/sr=1-7/ref=sr_1_7/103-1390974-9165457?v=glance&s=books
