I really like the tones you've achieved for those colours; they definitely complement each other well.
One thing I've seen that I quite like is painting the chestplate that the aquila sits on a different colour. These are the Iron Ravens painted by Paul Norton, a member of the GW Army Painter team:
You can see that he uses more blue on higher-ranking characters; while regular troops and their Sergeants have silver chests and thigh plates, the Ancient has blue arms, the Lieutenant also has blue upper chestplate and thigh plates, and the Captain has additional blue plates on his torso.
Aside from the use of colour to denote rank like that, Mk X armour has a lot of opportunities for that sort of colour choice; you could try painting the upper chestplate red, with either a gold aquila or another colour (since some chapter use aquila colour to denote company).
You mentioned you're not sold on the helm crest in red; since Mk X helmets are so similar to Mk IV, you could try painting the faceplate in a different colour, like the 'Eavy Metal scheme for the Betrayal at Calth Dark Apostle:
Edited by Ascanius, 21 March 2018 - 04:47 AM.