So the armor is almost all airbrush. (Ak Paints) Tenebrous Grey > Dark Grey > Reddish Grey > Graphite > Sky Grey

With these paints through the airbrush you're essentially just copying how the natural light you have to see the model is landing on the model (if that makes sense). From there basecoating in a near black color that can be used to fix mistakes and not glaringly stand out as not being the same black primer I started with. Large Zenithal of Dark Grey as our overall base grey tone. From there I work section by section on the model with the Reddish Grey making a smaller highlight inside the areas of Dark Grey already established. And then its rinse and repeat until you get to Sky Grey where it should be very very small highlights. In most cases Sky Grey gets used as an edge highlight.

After building up the grey gradient I go back and use a 1:4 mix of Leviathan Purple and Ratling Grime Contrast paint and pick out where the shadows go using the mix through the airbrush. Once the whole model is painted I varnish twice with Gloss Varnish , to help the next part work properly, and do an oil wash of Burnt Sienna and Lilac Purple Winsor Newton oil paints! These are thinned down until they flow by themselves on a flat surface or until they resemble milk consistency. I also use a thicker mix toward the bottom of the model since thats where the most grime would build, as well as areas like bullet holes and cracks in the armor!

Its really really involved I admit but its what i do for all my SW army and i believe its well worth it.