I remember that I once liked colouring books or, at least, I remember enjoying colouring with the kids I was babysitting. There was something satisfying in choosing colours and playing with texture and pressure, often while having a conversation. Colouring inside the lines is often scorned, but the lines make it easy to relax into colour.
My children had a different kind of childhood, and I remember only a few colouring books, mostly used as samples of how to draw superheros. My children learned to sketch early, and I encouraged them to take sketch books to picnics and parks and art galleries. They delighted in making the lines and sometimes even in filling them with colour.
If someone else gives you the edges, you will probably bump into them. You will probably find that your colours move over the lines here and there. You will probably feel constrained. The problem may not be lines. It may be lines that don't provide you with the shape you need.
Because when you make your own lines, you can be thoroughly, joyfully engaged in getting the shape and thickness just right. And if you choose to colour, you will do that thoroughly and joyfully, too. You will sketch and sketch, just bits here and there, preparing for a work that will expand beyond the limits of the page. You will begin to make sketches so that you can make yourself.
Even the sketches that fit in the middle of the page reach beyond it to the person who draws and the person who looks. The third dimension is the one without an edge.