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What exactly was the colour scheme of the Imperial Fists First Company pre-HH, I had always assumed it was their normal yellow scheme but have heard it mentioned that they all wore the same colours as Sigismund (black and white) which sounds like it could be true as he was the First Company Captain.

The Heraldry of Sigismund was black and white in his duty as a champion, not his scheme as Captain of the first company.
I'd guess yellow was always part of the ImperialFists colors.

They were yellow with just red helmets with a white line down the centre of their helmets

No. The red helmet was a sign of a marine under some form of penance as we saw in Know no Fear with the one Ultramarine Sgt bearing it with shame. It is only after his actions that the red helmet becomes a mark of honor.

Actually, the difference, is the fact you brought up Ultramarines.

IIRC, with Imperial Fists, the Red helmet is standard, and not rank. Unlike the Ultramarines, where post-heresy it became a mark of rank.

However, pre-heresy, it was a mark of shame, and then, should the marine in question emerge victorious, a mark of honor. But, thisonly applies to the Ultramarines, as the legions were scattered at this time period and did not have a standardized system for their armor ornamentation.

So, TL;DR, for the Imperial Fists, the helmet was always standard but, in the Ultramarines, it was a mark of shame, victory, or later after the Horus Heresy, a symbol denoting rank.

However, that is only how I recall it, I implore others to prove me wrong, so that this man is steered clear.