The WPP-owned agency won the top award, as well as a silver Lion, for its work on the campaign, which aims to promote cycling safety and draw attention to Volvo’s XC90 marque.
Grey teamed up with Swedish start-up Albedo100 to release a spray paint that makes surfaces glow in the glare of headlights.
The work included an online film and posters treated with Life Paint that reveal their message when hit by a camera’s flash.
AKQA London and its sister Shanghai office won the gold Lion in the same category for its "house of mamba" work for Nike Basketball China.
Grey London and AKQA were the two winning UK agencies out of a total of nine that were shortlisted for the Promotion and Activation category.
Nils Leonard, Grey London's chairman and chief creative officer, told Campaign he was "chuffed" after his agency received the accolade.
He said: "Grey London want to be a creative company, that people in the real world are glad exists.
"Partnering with the world’s most ambitious brands to play a meaningful part in culture is about creating more than just adverts, and Life Paint brings to life this philosophy.
"To win the Grand Prix at Cannes means a lot to Grey London, we're proud to help define our changing industry on this global stage."