If I may add my knowledge to this.

Skipping a lot of details.

1st, after Emps' death, the Custodes were reported to don "Black shrouds", they may have painted their armour black also, but it is never stated. We only know they now cover themselves in black shrouds, probably hiding the gold armour.

Additionally, what most people don't know, that incursion into the Imperial webway is still going in pretty much full force, all these years. This is why Custodes are relatively rarely seen on Terra, because most are actually down in the massive basement where this is happening. The ones seen are on the "Break rotation" where they are to rest up and their armour is fixed before going back down. This is possibly why some Custodes are somewhat lacking in armour, or have odd equipment, as their normal stuff is getting fixed.

BTW, why does the guy on the right have a CSM backpack? Is...it...HERESY!!!!!!