There really is something very therapeutic about colouring in. It's to do with taking the time out of your life to do something that has absolutely no purpose - it's watching great swathes of colour flow onto the paper, magically released from your pencils. It's making a vision of the world how you want it to be, not how it is or how it should be, but just how you want at that moment. It's green skies and purple trees and bright blue kittens. It's big sheets of creamy coloured potential rolled out in front of you, and the way wax picks up the texture of the grain. It's the twirly bits that come out of your sharpener. It takes no skill, there are no rules, so there is no pressure.
You are not being marked or assessed, there's no time limit or expectation of commitment. You can play around with what works, safe in the knowledge that it won't make one blind bit of difference.
And it is always incredibly satisfying, after a hard crayoning session, in tidying all those shots of colour away, carefully sorting them into rainbow order and running you hand over them one last time before bed.