I think that boat has long sailed without me, have you seen how many colouring books there are on the market? It's annoying as I did think of the idea about 4 years ago and dismissed it as I really did not think that adults would want colouring in books...
Now I dont think I could get a foot in the market and I am pretty sure that the artists that producing a lot of the drawings get paid peanuts for their work, even if I thought I could find the time.