The yellow was the legendary discontinued Citadel yellow ink. I also changed the paint scheme for the Fist emblem to just a straight Chaos Black with built up Skull White highlights.
I made all the hatches and the weapon turret interchangeable with a generic top hatch so I could economize on use, the Razorback transforms into a Rhino! Waste not, want not!
Here's one of the Forge World hatches I got months and months ago, immortalized on this here Razorback. My last paintjob on a Razorback had the hatch itself as black, the ring in red, and the fist in black. I much prefer this rendition, it's brighter and it follows the actual color scheme of the real Fist emblem.