I've recently treated myself to a Repulsor Executioner tank for Warhammer 40k. The guy on the box is clearly an ultramarine (see his left pauldron) but his armour is red.
My understanding is that Sergeants and Officers in the Ultramarines (and most codex following chapters) paint their helmets red following Guilliman's command at the Battle of Calth when Thiel (a marine under censure) turned out to be right about
the possibility that astartes could end up fighting astartes
However, this marine appears to have completely red armour. Is there a canonical reason for this or is it simply because the 'Eavy Metal decided it would look cool?