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.
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
SP Lark Light Grey