The adult colouring book craze is growing quickly! Studies have shown that colouring is an excellent activity to reduce stress. The studies are true: more and more, adults are turning to colouring books to help them relax. But, you can’t colour without the proper tools and for that, Tombow USA has you covered! The Tombow Dual Brush Pens are one of our most popular products, and they are amazing for use with colouring books. .