If you go the way of accepting Nick Kyme's Tome of Fire series as fluff ~(Which I have to say I do - simply because he's fleshed it out a bit), then Techmarines have a red right pauldron and red helmets.
IMHO, I would avoid a red helmet simple because it could be confused with a Codex-compliant Chapter's Sergeants, and it could equally be confused with the fluff for Salamanders Techmarines if you field them together. Again,
IMO, I'm not so keen on the black armour - simply because
IA ten, and the Tome of Fire series, presents veterans as being in standard colours (Although I remember a piece of old fluff stated that they reversed the pauldron colours).
All that said though, it's you model and what you've done so far, aside from my personal preferences, is very good! Keep up the good work!