I like the idea of black, but perhaps with some red detailing.
Im thinking of putting him on a bike so need to find a servo arm for him.
Judging by the 'current' Dark Angels, I would say Red with Black details. I had a quick scan through Fallen Angels to see if it mentioned Askellon's armour colour, but couldn't see it anywhere.
Predominantly Black with minute Red detailing.
I think with the DA's being the first and such an uber proud legion, even the mechanicum would have to take a backseat when their pride is involved.
'...as black as the armour of the warriors he would come to lead'