Heraldry in a virtual sense had not been developed in the 11th century yet. Seemingly it came into widespread use first during the 13th century and reached its peak in the 14th and 15th century. However, one could suggest that shields were used before to show some kind of allegiance. We simply don't have reliable evidence. So basically it's up to you if you want to give your Normans a bit of uniformity.
For my taste, Medieval units look 'more realistic', but overall simply better if sporting a lot of different colours.