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

Set Contains 

AX-033 DS Surface Grey 

AX-007 1975 SP Lettering Grey 

AX-009 1975 SP Lark Light Grey

AX-008 1975 SP Lark Dark Grey

AX-003 Dark Grime

Other recomendations are

Rust

Weathered black

Roof Brown

Foundation