feature nail
n. A fingernail with an applied color, pattern, or shape that is different from the other nails.
Examples
2013
Commonly known as the "feature nail," this trend is far from over, thanks to the ever-increasing interest in nail art. The fashion world has been taking nails and nail art very seriously in the past few years, as seen on international designer runways. The trend of painting, patterning and/or embellishing one nail differently is now being embraced both by celebrities and by the mainstream.
—Bahar Niramwalla, “What’s a 'feature nail’ and how do I wear it?,” The Globe and Mail, April 13, 2013
2012
When it comes to killer nails, Lady Gaga's latest manicure could actually do some damage. The pop star stepped out with a huge sharpened talon on her finger on Saturday morning. The five-inch feature nail looked seriously scary, as she flashed it at photographers.
2003 (earliest)
The nails were a funky, three-dimensional retro stripe with colours that followed the collection's sassy street theme.

"Any Gold Coast girl can do a feature nail easily to match their clothing colours."
—Rachel Syers, “Fingers do the talking,” Goal Coast Bulletin, May 20, 2003
Notes
Kudos to Karen H. for fingering this phrase.
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