HI Loiero, I always use a complement colour on my bolt guns/weapons in general.
For this Iron Hand I went with a damaged white, this is so the eye is drawn to the white first and then the rest of the model.
How ever some Painters prefer the armour that the marines are wearing to stand out more (All
GW marines for instance).
In the case of your red armour I would use either a white or black bolt gun.