Nail polish has come a long way from just being one color - crackle nail polish, decals, Minx ... so it idn't surprise me at all when I began seeing friends painting their ring fingers a different color from the rest. The overall effect is subtle - like wearing a ring on that finger, it stands out - but depending on the color choices you make, can be bold as well.
I personally like the subtlety. I decided to try this today with my Essie "Demure Vixen", a mauve-beige-violet, and Urban Outfitter's "After Hours", a multi-colored pure glitter polish. Staying within color families is my favorite way to do this trend for now - for example, orange with copper or gold. The "After Hours" has just enough violet in it to look relevant, but still pops. However, I have seen girls do this with all neon shades and it looks just as cool!