Quick little tip. I prefer to use white gouache for adding a touch of white to my watercolor paintings.
I squeeze the paint fresh from the tube into a little ceramic dish. I prefer to use it fresh from the tube, but often times I end up squeezing out way more than I need.
I have found a little spritz bottle filled with water works wonders for re-wetting the white paint. I spray the paint with water a few minutes before I am ready to use the white. It softens the paint.
Using the spray bottle, insures that I am only wetting it with clean, clear water. I also make sure I use a clean brush. This way I do not contaminate my white with another color.
This works great whether you have white watercolor paint, which I don't recommend at all or white gouache (basically an opaque & matte watercolor).
For more about white paint in watercolor painting, read one of my old Tuesday's Tips: