The green light: Livia Firth and her Green Carpet Challenge

The headquarters of Eco-Age, Livia Firth's consultancy business, are set back slightly from King Street in Chiswick, West London, behind a whitewashed facade. Inside, there is a large, open-plan space, with a giant bird's nest complete with painted eggs near the doorway. It is a prop left over from an event Firth organised last year to launch her first Green Carpet Challenge (GCC) capsule collection - which involved five high-profile British designers and had the support of American Vogue editor-in-chief Anna Wintour and Natalie Massenet, of online luxury fashion store Net-a-Porter.

"Managing ethics and aesthetics" is Eco-Age's motto. Firth set up the company with her brother, Nicola Giuggioli, in 2008. Her husband, actor Colin Firth, is a partner. They started out with a shop on Chiswick High Road, selling eco-friendly paints, sustainable homewares, bamboo towels and wind-up torches - useful things that have minimal impact on the environment. If you were lucky, you might even have been served by Colin. They also ran a consultancy helping businesses become more sustainable. Projects included a solar-powered recording studio for singer KT Tunstall.

In May last year they quit the retail business and moved to larger premises to focus on consultancy. Giuggioli, who completed a master's in sustainable business at Italy's Roma Tre University before moving to London in 2004, is the chief executive.

The company's office looks like that of a busy start-up. Livia Firth's desk has bookshelves behind it, and a pinboard to one side is covered in a mix of items that reveal much about her. There is a special report on London fashion shows by Suzy Menkes, from the International Herald Tribune; a picture of the Firths together, torn from a magazine; a glamorous shot of a model in a beautiful green ballgown, along with some other fashion images; and a newspaper spread showing the aftermath of last year's Rana Plaza factory disaster, in Bangladesh.

Firth, 44, has become an expert in dressing herself stylishly, as well as ethically. Slim and very neatly put together - dark eyes, chiselled cheeks, hair pulled into a sleek side ponytail - she is about as far as you can get from the stereotypical hair-shirted, hempseed-eating eco-activist.

Her most important attribute is her ability to play the game with luxury fashion's power circle. As a regular on the red carpet, attending premieres, charity dinners, the Golden Globes and the Oscars with her husband, Firth came up with the idea of using her profile to champion sustainable fashion. For the Venice Film Festival premiere of A Single Man, in 2009, she wore a dress by her friend Orsola de Castro, of the upcycling label From Somewhere. Firth also sought the help of a friend and mentor, ethical-living writer Lucy Siegle, and, in December 2009, they came up with the idea of the GCC.

For the first GCC, at the Golden Globes in 2010, Firth had red-carpet reporters slightly mystified when she turned up in a repurposed Christiana Couture wedding dress. For the Paris premiere of The King's Speech, in January 2011, she wore an outfit made from one of Colin's old suits. And when her husband won an Oscar the following month, she wore a dress inspired by one her grandmother had owned. It was made by Gary Harvey - a relative unknown in international fashion - by patching together 11 vintage frocks.