Actress Joan Collins was in London yesterday to launch the first-ever film version of "Joseph and the Amazing Technicolor Dreamcoat," the musical that has enjoyed on-stage success for more than 30 years. In the movie, Collins plays Mrs. Potiphar, a woman who seduces Joseph, who is played by Donny Osmond. Collins, who arrived at the launch wearing stiletto heels and bright-red lipstick, admitted to PEOPLE that she has never seen the musical before but was invited to join the production by her friend Andrew Lloyd Webber, who wrote the music. "He sort of persuaded me to do it, even though it's a smallish role," she said. "Small but impactful." But she jokes that she regretted taking on the role "when I saw the costume" which she describes as " the most unforgiving costume I've ever seen." Next up for Collins is a role in next summer's big screen "Flintstones" prequel (she plays Fred's mother-in-law) and the film "Showtime Grenada," based on a script she wrote.