Watercolors can add a subtle background to an art journal page. This is a very serendipitous way to add a layer to a background. Here is my purple tag background after stamping with watercolors. You can see it in a finished journal page here.
Step 1: Start with a prepped journal page. I prep it with either gesso or acrylic paint. The important part is for the watercolors to float on top in the next step.
Step 2: Slather on watercolors.
Step 3: While the watercolor is still very wet, use an uninked rubber stamp to REMOVE some of the watercolor. Let it dry.
Here is what the blue looks like dry-
Some other examples: