Rubric is post horus death, so basically any
TS you have faffing about during the heresy/great crusade would be red
As to the
TS being involved in the heresy: Unless fluff is changing/has changed, they basically sat it out and played neutral. In part because bitterness (Ahriman points out in one of the short stories that they were betrayed by the Emperor, but Horus had nothing to offer them) and in part because Magnus didn't want to be uninformed again (pointed out in the same short story and when Khan saved him from being split up and stuck on Prospero)