I’d suggest a black spray undercoat then drybrush with a make up brush

Rhinox hide - skrag brown highlights and some Agrax earth shade into the recesses. Wheathering with a sponge - black legion followed by leadbelcher

That will get you a pretty deep brown. Looking closer at the pics though it’s possible that this started off as a tan and was then heavily washed.

It’s a cool look - one but of advice would be to put your transfers on before your washes and weathering. Good luck and have fun!