In watercolor painting the best way to have white in a painting, is to save the white of the paper. If you want to create something with a specific white shape, sometimes it is best to use masking fluid. There are a variety of brands of masking fluid and each brand has tinted theirs a different color. The brand I like the best is Pebeo Drawing Gum. It has a gray tint to it. Which can be easily seen as you are painting.
I have a brush that I use only for the masking fluid. I have labeled it with a piece of tape on the handle. It is good to gently shake the masking fluid bottle a few minutes before you plan to use it, and let the bubbles settle. I wet the brush with clean water and then swipe it across a small bar of soap, then I use a tissue to blot off the excess soap. The brush is now ready to be dipped into the masking fluid. By soaping up the brush first, you will save the hairs on the brush. If you are masking a large area, it is a good idea to go back and re-soap the brush again and again.
Allow the masking fluid to completely dry before you start to paint. When your painting is complete and dry, use a rubber cement pickup (the square object in the photo) to remove all of the masking fluid.