Question: What do the different colors and shapes of Crew tags mean?  

Answer: Two stripes on a Crew tag denotes a Founder Crew, which is a special designation granted to any Crew created before May 15.  White angled tags mean that the crew is user-made, while white tags with a straight edge are for public, Rockstar-made Crews.  Yellow Crew Tags are shown on tags for the Crew you currently have chosen to represent in Multiplayer, which can be changed in the Crews menu within Multiplayer.