Blue was exclusive to AT&T. Purple was exclusove to Sprint. And brown, for some odd reason, was the exclusive to Verizon.
AT&T might have gotten a red one too.
All 4 carriers got black and white from day 1.
You can get the colored battery doors from any other color and put them on yours. My black t mobile S4 has a Sprint purple back.
Here's them all to see...
Sent from your phone. You should be more careful where you leave that thing.