With mindfulness the buzz word of the moment, colouring in is an easy way to calm the mind and occupy the hands. Speaking at a mental health workshop in 2009, author, speaker and communication expert Mark Robert Waldman explained that active meditation focuses attention on simple tasks that require repetitive motion. Concentrating this way replaces negative thoughts and creates a state of peace, and many people who have a difficult time with concentrative meditation can find this easier. This gentle activity where you choose the colours to create your picture and the repetitive action of colouring it in focuses the brain on the present, blocking out any intrusive thoughts.
Here's a couple links to get you started!