Connor
A pin wash is a very thin wash of paint which can be used to highlight and shade both vehicles and figures.
I use mostly black and raw umber for clothing and faces. I mix my own using acrylic paint and thinners although I believe that you can buy washes already made up. The important thing is that the wash has to be very thin so that it flows smoothly using capillary action from your brush into the nooks and crannies to be shaded. Several washes can be applied until you are satisfied with the look of the figure.
A word of warning do not saturate your figure or you could lose paint that you have already applied as the thinner tends to take the paint off and make sure the wash has dried before you attempt any other painting.
It takes a bit of practice but worth doing.
Cheers
Gerry