Supermodel Heidi Klum, who has posed wearing only multi-coloured make-up for a cosmetic advertisement, says she is happiest when naked.The 38-year-old, face of Astor, an European cosmetics company, wore only make-up on her body for the advertisement, reports dailystar.co.uk."I am a nudist. I have never had a problem with my body," she told allure.com.