Since the pistol with the rows of glowing lights is small, but also intended to be a Flamer, which tend to be big guns in the modern Warhammer 40K Universe, Zwakke was painted in bright colors consistent with fire. Except for his right arm, which has an inconveniently large Las-Pistol, and was therefore painted in dark colors to make it easier to ignore.
Spc Gravensteen is assisted in his attempt to emphasize his Flamer by his rat, who has the fine lustrous red fur that can only be achieved by a steady diet of Eldar Fire Dragons.
Gravensteen is another Squat where none of the converting involved removing bits of the mini. Aside from Col Iron Beard and Vox-Lieutenant Lawaaierig, this command squad is made of Squats sculpted by Bob Olley featured in White Dwarf 108. Most of them were well-behaved troopers with Las-Guns with padded Imperial Guard style armor and other acceptable weapons, and were put in the Blue Catalog and kept in production. Others were dropped as too strange looking or having too many guns. Gravensteen went out of production in 1991 or so, as did 3 more of the upcoming Squats in this unit.
Like Spc Poelgeest, who remained in production, Spc Gravensteen has been embellished with an Iron Cross from the Black Templars, hopefully for the obvious connection.
The main conversion for Zwakke is as subtle as an Ork, flame-like spines extending from his waist up along his back to his shoulders. They are from the Dark Eldar Warriors helmet bits, which you probably thought were useless until now.
The flame-spines give Gravensteen a bit of a dinosaur look to him. Which in 40K terms, is exactly what he is.
More Fun with the World Eaters. So cute when they try to be sneaky.
From the World Eaters Recipe Codex: No matter what color the outside of the Space Marine is, they all taste the same on the inside.
Col Iron Beard and Company