interesting...I wasn't sure. I've done a similar type of colouring in illustrator.
I do clean line artwork, then I scan it and either redraw it in illustrator or use the live trace tool in Illustrator CS. Then I make that a top layer and add layers of colours underneath. It works in a similar way and the gradients you've used in the background can be done with the gradient mesh tool. I like working with the vector because I can make it any size I want and not have to worry about how it's going to look when it's printed.
But hey that's just me. And yours looks good anyway