This morning Bruno Vaiano asked a question in the flickr military group that got me interested. Real military vehicles often have a weathered or somewhat dirty look, certainly the all white vehicles often used by UN units, and Bruno asked about ways to recreate this effect in LEGO.
Short of using dirty bricks or using paint, I felt this is something hard to do using just LEGO bricks. The only weathering that I've done involved my F-14A Tomcat, where the two colours I used lie close together (blueish grey and old grey) and the effect is so subtle you'll be hard pressed to see it in a picture.
John Lamarck answered with an interesting method that does involve making your bricks dirty. He burns the end of a cork using a candle (an excellent excuse to open a bottle of wine) and rubs the cork onto his LEGO leaving a fine layer of soot behind on the bricks. According to him it can be removed using a tooth brush and some water.
I'll take his word for it, but the result looks pretty good. The aircraft itself is a model of a Junkers EF-112, an aircraft designed during WW-II that never flew. It was John's entry for this years flickr military build contest and ended up winning him a well-deserved third prize in the 'Alternative Reality WW-II" category.
Try this method at your own peril. I will not be held responsible for you burning the house down playing with flames and corks after drinking a bottle of wine!