I think steel as the primary colour is definitely the way to go - they are the ‘Steel Beasts’ afterall. I reckon blue would be a bad idea, as then the colour scheme is too reminiscent of the
recent colourscheme invented for the renegade Astral Claws:
I dislike the idea of a black and metal colourscheme, as we’ve recently had the colourscheme for the Warmongers (Legio Crucius) officially set in the recent Forge World books as
mainly black (with white and gold detailing).
The old Titan Legions rulebook definitely had a symbol for the Titan Legion – my copy is packed away somewhere, but presumably that is the one on the wiki. I think should be adopted as their symbol, to fit the existing background and because it looks quite orkily close combat orientated with those axes.
The Titan legion is specifically Orky, so I reckon having them look slightly orky would be good. Steel with green trim/detail is my suggestion as green is evocative of orks in general. Red could be another reasonable possibility instead or as well, but red is somewhat over-used on titan legions.