Traditionally in watercolor painting, white is the color of the paper, not the color of the paint. In other words, you achieve the most vibrant white by not painting. Additionally, adding white to a color doesn't make it brighter; it makes it cloudy. For these two reasons, using white is frowned upon by some purists. However, white isn’t inherently good or bad; it’s simply a tool. If you know what you want to achieve, and you can best achieve that by using white, that’s the color you should use.