As DrtV posted, the milites would most probably have been in coif and helm with no hair on display, as would a proportion of the spearmen. So you only really have to worry about the rest of the foot.
Given the diversity of the army and the broad genetic base of eleventh century northern France/low countries (for want of better names), a spread of hair colours makes sense. I'd probably go, two blonde, two ginger, two medium brown, and four dark brown.