Maybe to draw attention to the ring finger, if there is no ring there, its a good way i suppose to let other possible partners know you are available.
I asked one of my customers at work about that (on the index or middle finger), and she told me that the different color indicated...the trigger finger! Kinda unworkable for the ring finger, tho...
It's just an excuse for us to use two different colors like we're 5, but not look completely childish. lol. There's no significance in choosing the ring finger.
I've never seen that. If it's a new fad it's not one anybody I know has caught on to yet.