It is really easy in Gimp to take a line image as in a colouring book and make it into individual bits. You can also use the airbrush tool to get the lovely fade effect you get when painting with an actual airbrush - without the mess!
Like this quick example.
In this video I show you how to get rid of the background colour and how to select individual pieces to copy and paste where you want them.
If you need to, you can add a stroke (outline) to your selection when you copy it. Go to edit 'stroke selection' and choose the line width you want. Like this
It would make tracing easier in Studio, when you have a variation of colours in the fill as I do here.
With these features mastered, the possibilities are endless!