That's a question I've received a few times. I like doing an umber wash at the end because the wash will settle into the battle scars and other places to create a dirty look/feel. It gives everything a nice tone.
I apply my wash with an airbrush, which is maybe out of the ordinary. The reason for this is so I don't get any brush strokes on my helmet. It is just a cleaner application, IMO.
Here are a couple links for the wash I like to use:
https://www.hobbylinc.com/vallejo-umber-wash-17ml-6-pack-hobby-and-model-acrylic-paint-73203
https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&field-keywords=umber+wash
Here is a side by side so you can see the before and after results of a wash.
[ATTACH]69074[/ATTACH]