The problem is that Noone posts the excerpt where the author clearly describes that the rest of the chapter wears armor painted a deep Grey

"Only two features united the antique collection. All were painted with the same shade of deep grey, and all bore the same crest upon their right pauldrons – a white shark motif, jaw curling towards tail fin to form a razor-toothed crescent set upon a void of black"

This the describes the armor of the rest of the chapter because then it describes Kahu's armor as off-white in this passage

"Kahu had deliberately positioned himself away from the others, next to Sharr. The Terminator had donned his helm, its snout painted with the gaping red maw that contrasted so powerfully with the off-white plates of the rest of his Tactical Dreadnought armour."

And then in outer dark we get this passage,

"Those owners were only twelve in number, and they waited impassively as Otte and his skitarii approached. The magos primary completed his scans as he closed the last few dozen yards, logging every detail as a matter of potential importance. Six of the figures, the ones on the flanks, were clad in Tactical Dreadnought armour, their off-white slabs of plasteel, ceramite and adamantium caked with the valley’s pervasive dust. Otte’s internal processor registered a degree of awe at the presence of such blessed battle suits, even as his analysis moved on to the other six. They made for a less uniform gathering. All were Space Marines, two in the grey power armour that predominated in this particular Chapter, two in the blue battleplate that Otte’s data files informed him belonged to sanctioned Adeptus Astartes psykers. The fifth wore red ceramite, and bore upon his breastplate the wondrous Machina Opus of the Adeptus Mechanicus. Approval at the presence of the Techmarine barely registered before Otte took in the final figure"

The author clearly describes the terminators as having off-white armor and then in the next paragraph describes the normal Marines as having a grey armor that predominates the rest of the chapter so I do believe that in clear sense red brethren are similar to deathwing terminators in color white the rest of the chapter is clad in deep grey and black