Spray the model in Abaddon Black
1:1 Water/Paint with Mephiston Red and stipple it all over the armor/model using a drybrush, don't worry if you can't reach everywhere with it, the darkness is nice for the grimdark vibes- 1:1 Water/Paint with Evil Sunz Scarlet , again stipple it and focus on the parts that the light reaches, the shoulders, head, etc