Actually usually the painter are (were, at least) left free to interpret the general theme as they wanted. After all, most titan legions are 10 thousand+ year old, it is only normal that their paint scheme is varied, according to the history of each millennia, each campaign, even each titan or princepture, etc...
I personally think that strict adherence to a paint scheme which was, after all, made-up (or even re-made-up in case of FW interpretations) is too much restriction.
The universe is vast, specially the 40k one, and everyone should feel free to re-interpret colour schemes and consider them more guideline than anything else; our models are not historical miniatures.
I Like to think of officially published illustrations and photos of painted minis as inspiration rather than restriction, although I have no doubt many an anal retentive teenage wh40K player wouldn't understand a bit about that feeling (These are not Dark angels ! Company marking are not on the knee pads, they are not TRUE Dark Angels ! Mum, tell him !).
Obviously, even the official painters, commissioned or even full time 'eavy metal people all have their own opinions too about what colour a shaim-ann eldar or an Ultramarine should be. Over recent years, even some core-game center army such as space marines chapter schemes have evolved.
The Painters behind the FW remake of the War griffins and Legio Astorum allowed themselves quite some freedom compared to the original, and it is probably what made those re-interpretations so nice.
So feel free to re-interpret as you wish. See the few official painted minis and illustrations for L. Ignatum here on lexicanum : no two paint schemes are the same :
http://wh40k.lexicanum.de/wiki/Legio_Ignatum(For example, I quite like BlackLegion interpretation on last page)
