All you'll need is a good old waxy crayon and some watercolors. I personally like to use the often over looked white crayon on my pages to hold blank areas, but experiment to your heart's content with this!
Now that you have your crayon and your watercolors, grab your journal and doodle something with the crayon. Press hard to leave good solid waxy marks. If your pages are on the thin side, go over the same lines gently a few times. Good build up is key!
Then, take your watercolors and paint right over the entire area of page you want to be that color. If you're only using one color... paint the entire page.
Did you notice that your paint isn't sticking to particular parts of the page? Namely the parts you drew on with your crayon? Wax resists water, so while your paint may bead up on it, a gentle swipe with your finger, cloth or a dry brush will clean it up. This is a great method for keeping white space on a page, or for building up mediums.
You can use this technique for various effects, such as line work or blocking out large chunks of your page. If you only push gently on the crayon as you go, the lines will be less bright as there is less wax between your paint and paper and it will still bond to an extent.
Here are examples of a few pages I've used this technique for:
As background mushroom stems...
As the focal point of my design. If you click this one, you can view it larger and see where I pushed harder or more gently on the crayon as I drew.
Go ahead and give this a try. Let me know what you think!