The At-AT walkers varied form in size and scale and paint techniques, they tend to have a lot of stains rathe than solid colours, thinning down paint is a good way of recreating the look but makes it harder to establish exit colours. Below is a basic set that should get you through most AT-AT.

Set Contains 

AX-007 1975 SP Lettering Grey (for base coat)

AX-008 1975 SP Lark Dark Grey 

AX-018 1975 Concrete (ROTJ Walker base)

AX-006 1975 Light Reefer Grey

AX-011 1975 Engine Black

Other recomendations are

Rust

Weathered black

SP Lark Light Grey

Foundation