The Blood Angels in the artwork certainly display a variety of markings, and do not appear to have used the same helmet system they did today.
As you say, the majority of Assault Squads have red helmets, which certainly seems to show that they did not use yellow helmets to designate assault squads as currently. In addition, the assault squads seem to have a variets of different insignia on their shoulder pads - there are examples of the 40K Assault arrows (Lorator Assault Squad, p. 387), blood drops (Orexis Vetaran 'Tactical' Squad p. 174 - good old Collected Visions
and their labelling mistakes) and even what looks like it could be the tactical arrow, but is obscured by the border of the picture (Traitor's Gambit, p.251). Tactical Squads also seem to pretty much entirely in red helmets, often with the tactical arrow on their pad (Vallerus Tactical Squad p.68, Clonatus Tactical Squad p.173). Frustratingly there are no examples of Devastator squads, but I would assume given the lack of yellow helmeted assault squads that they would have red helmets. Vetarans seem to sometimes have gold helmets (Orexis Vetaran 'Tactical' Squad again - actually appaears to be a vetaran assault squad, and Fevreus 'Tactical' Squad- which carries a banner so is probably a vetaran or command squad) although occasionally red helmets (Emiror Vetaran Tactical Squad, p. 174) and occasionally have blooddrops in yellow/gold or white (Orexis and Fevreus).
So, how to designate BA squads? A couple of options seem to best fit what information we have. One is to use standard Codex markings (which certainly seem to have been used instead of helmet colour by the Pre-heresy BA) and red helmets for tactical, assault and devastator squads. Vetaran squads can then be represented by gold helmets, perhaps accompanied by a yellow blood drop (vetaran assault squads) or a white blood drop. Another alternative I suggested here
is to use a coloured blood drop to designate squad type - yellow for assault, blue for devastators, and white for tacticals, with gold helmets designating vetarans. Both of these systems have the advantage of being distinct from their 40K iconography and thus helping to mark them as pre-heresy, and can be at last partially justified by the examples in the artwork.
All the standard codex squad markings were already in use going by the artwork. The BA alone provide examples of the tactical arrow and the assault cross (see above) and there are examples of the current devastator marking as well (Van Kordal Heavy Support Squad, p.317, for example).
I'd be inclined to avoid using all white armour personally. White armour seems only to be used by Sanguinary Priests among the Blood Angels today, not used even where the Codex suggests it (white helmets on veterans, for example). In addition, despite there being several examples of pre-heresy BA vetarans in the artwork, none have white armour or even white helmets or shoulderpads. As you say, you can potentially justify it given the size of Legions and non-standardisation among insignia used, but it doesn't seem particularly 'Blood Angel-ly'.