Different coloured dog tags when knifing
Solution 1:
Its based on the persons level.
Bronze: Knife kill a player with the ranking 1-21
Silver: Knife kill a player with the ranking 22-45
Gold: Knife kill a player with the ranking 46-50
Source: http://loot-ninja.com/2010/03/25/battlefield-bad-company-2-dog-tags-explained/