Feeding your example sentence to Google leads me to think that
heraldic definitely does not refer to colour in the way you propose.
Heraldry refers in general to to use of both colour and shape (although colours are often represented by different hashes).
On the Bayeux Tapestry, there are coloured shields (literally), but they do not represent the bearers in any heraldic sense of the word. So the sentence should not be read:
The Bayeux Tapestry of 1068 shows designs in [heraldic] colour but not of heraldic form.
The Bayeux Tapestry of 1068 shows designs in colour but [the used designs are] not of heraldic form.
Your interpretation of the sentence would have been possible if the preposition
of would have been absent, or replaced by
in to mimic