From the 1973 exhibition we have a good description of Queen Alexandra's purple/petunia coronation robe.
So it seems to me that the painters of the wonderful paintings here adjusted the colours when portraying the coronation robes of Queen Alexandra and Queen Mary.

Further, I don't think Queen Alexandra wore a crimson Parliament robe to the Abbey.
It is difficult to prove that, I admit. Here is her Parliament robe with a full cape, and her departure for the Abbey.
Perhaps those two "lapels" folded back.