i consider myself a cheat ,
spray grey primer, then wash all over,
pre mixed colours, like armour with brown to give it a rusty edge ,
then use mass washes and a hair dryer to speed up.
Art Pen to dot the eyes,
highlighting with a small brush speedily ,
then also the black BG i use to photo the model is really kind to the shading, and absorbs excess light nicely, so the pics always look ok.
i use GIMP Photo Tool to adjust the brightness and Contrast automatically
etc
it's fast