DC Comics promoted the launch of the Blackest Night mega-crossover by giving away Black Lantern power rings. This was a hugely popular promotion with Blackest Night #1 selling out and going into multiple printings. I admit I bought into the hype and subscribed to the main books just to get the power ring.
It was only natural that they decided to release the power rings from the entire spectrum of light - red, orange, yellow, green, blue, indigo and violet by pegging the orders to various Blackest Night tie-ins. Retailers can order the rings based on orders of Blackest Night #5 (50 green rings/50 copies) and several tie-in issues: Justice League of America #39 (50 red rings/50 copies), Booster Gold #26 (50 orange rings/25 copies), Doom Patrol #4 (50 yellow rings/25 copies), Adventure Comics #4 (50 blue rings/25 copies), R.E.B.E.L.S #10 (50 indigo rings/25 copies) and Outsiders #24 (50 violet rings/25 copies). This probably resulted in some odd sales statistics for books like R.E.B.E.L.S, Outsiders, Doom Patrol and Booster Gold which have never sold well traditionally.
The rings are made of soft plastic and are one-size-fits-all. I did not purchase all these books but managed to score an entire set including the white power ring (tied to the follow-up series Brightest Day #1) and another Flash ring (tied to the release of Flash #1) by placing a minimum US$50 order with Mile High Comics who were giving a complete set as part of their freebies promotion. This giveaway was also immensely popular and the sets were given out within two days. A search on eBay indicates that each ring can probably be auctioned for a few US$.
There is also a full set available from DC direct, which are made of ceramic and are part of the Justice League Trophy Room set. The rings contain an LED that lights up when slipped onto the finger. I do not own this set though.
7 Lantern Corps rings - MISB
Black and White power rings - MISB
Flash ring - MISB
Blackest Night #1-8 (complete)
Blackest Night #0 which was a 2009 Free Comic Book Day release and served as a preview to Blackest Night