Here everything you need to know about the gratitudes :
http://www.spacetimestudios.com/showthread.php?106638
Not sure how you have two vet sets, either.
Silver 2 years shield is as probably already guessed - playing PL for 2 years (your account needs to be 2 years old)
Red 3 years shield is for playing PL for 3 years.
And the blue PL shield is for playing PL for 6 months.
http://www.spacetimestudios.com/showthread.php?106638
And before someone in prepuberty trolls this thread and everyone is senseless raging - PL came out in 2010.