Watercolor is a unique medium apart from oils and acrylics in that you do not use white paint.  To make a color lighter you add water to watercolors.  To show white areas you leave the paper white.  This method in watercolor painting requires some pre-planning because once you paint a color you can no longer get the white back.  Therefore, you must be aware of the white areas before you begin your painting.  The colors in your painting will be painted around the white areas.  If you have very small or intricate areas that need to stay white this process of "painting around the white areas"  can be tedious and difficult.  There is a handy tool that many watercolorists use to alleviate the difficulty of painting around the white areas.  It is called masking fluid.  Masking fluid comes clear or tinted.  What you do is "paint" the masking fluid on the areas on your watercolor paper that you want to remain white.  The masking fluid will dry and create a barrier over the white areas that will resist the watercolor paint that you paint over it.  With that in place you can freely complete the painting of your picture without worrying that you will accidentally cover the white areas.  When your painting is complete or whenever you are ready you will remove the masking by gently rubbing your finger over it.  Your white areas will appear and you can then work more on the painting if you need to.  Masking fluid can also be used over an area that you already painted if you need to protect that area from other colors.  Masking fluid applications come in many forms.  I particularly like those applicators that are very fine and allow you to "draw" in on in a fine line.  For larger areas there are applicators that will help with that.  Be careful of using a good watercolor brush  to apply masking fluid because it will dry on it like glue and ruin your brush.  In a pinch you can use a toothpick to apply it or another disposable tool.  has some great choices for masking fluid.