Paint recommendation for painting the edges of books?

I’m into book edge painting but I’d like to incorporate an airbrush for the background to get smooth clouds/gradients as I struggle to with with gouache or watercolour (the paints I use to do the details).

I’m not looking to do anything more detailed than a nicely blended background. I do need the paint to leave relatively thin coats as when the book is unclamped and the pages separated, the paint can flake if it’s too thick. Based on my use, are there any paints you’d recommend? I’d prefer to get the most inexpensive option I can that will still work for my purposes since I spend more on quality paints for the hand done details.

Any help is very much appreciated!