"I used to do all my work in black and white, and some of my clients used to joke about how they would like to take them home and colour them in," she said. "I was then asked to do a childrens' colouring book, and I said 'how about doing an adults' one as well?'. They were a bit tentative, but eventually they went for it, and printed 13,000 copies that sold out within a few weeks.