If you're refering to the 'historical' Dark Age Arthur, then the heraldry is anybody's guess.
Mounted Gododdin warriors supposedly carried 'white' or 'limed' shields.
If you're refering to the 'image of Mary' Arthur is said to have carried, that is a later interpolation, since historically when Arthur is supposed to have flourished, the cult of Mary was just a 'kickstarter' in the Eastern Mediterrrean.