With Star Wars models it's the painting that can really make or break a build. Unlike most of the Imperial vehicles in the original trilogy, the AT-ATs were pretty heavily weathered. I started with a good coat of Tamiya primer from the spray can (love this primer!). This is a pretty blue-ish grey, but I let a little stick around for color variation. I mixed a custom grey color with black, white, Sky Grey, and a few drops of a dark blue (all Tamiya). Made up a light and dark version of this, then sprayed all the panels on the model with the dark version, spraying from the panel center to leave a little bit of the primer showing at panel edges. After this I hit a few panels with a much darkened grey, either masked or hand shaded. Then I masked a few sections with some liquid mask applied with a sponge for chipped paint. Here are pics of this process.
After this I sprayed the lighter custom grey over most of the model, lightening the darkened parts and leaving some minor color variations throughout. I should add here that I love my new Iwata gravity feed, thanks Santa (me)! After everything was dry I removed the liquid mask, which I hate doing and wish there was a better way.
After that I went after it with the oil paints, primarily mixes of blue, black, brown, and occasionally white for lightening. I got the basic technique from amazing modeler FichtenFoo (Michael Ficthenmayer). The basic process is that I wet an area slightly with thinner, then use various brushes to apply small amounts of oil paint to tint panels and create streaks. I love this weathering method, you just keep pushing the color around until you have the look you want. You can concentrate on a whole panel to tint it, create shading effects, leave the paint kind of concentrated for drips, etc. Lots of variation for not a lot of work and I find it very meditative.
Hey, there's that ILM "universal greeblie"! (Revell Cylon Raider next to my scratchbuit AT-AT greeblie)
After all the oil paint was dry (overnight) I went over some areas again, waited another day. I wasn't happy with how the big chipping on the side panel came out so I sanded off and re-painted that section. Another night to dry, and I used powdered pastel chalks (black and white mostly) to enhance some of the streaks and burn marks, and add some more color variation. For some reason it's easier to lighten a panel with pastel than oil paint, so the white was useful here. The pastels are much less forgiving than oil, you have to take care where you apply high contrast powder!
Looking pretty good! Now we'll see how I built the diorama.