This step by step is for minor bullet pings on Armor. My base color is unbleached titanium, which looks like it falls somewhere between screaming skull and ushabti bone in GW paint. The brown is burnt umber, it looks like it could be between rhinox hide and mournfang brown.
1. Rough spots of brown laid down with irregular spacing. On darker or colorful armor I would try gray or ledbelcher on this step depending on the look you want.
2. Smaller spot of black (thinned 1:1 w/ satin glaze medium) put in middle of brown spots.
3. Thinned base color dabbed on bottom edge of black spot.
4. Another dot of black put in center of mark. Dot of white put underneath black. This is to pop the contrast a bit.