When painting in watercolor, the white in the painting is usually the white of the paper. Often times we forget to use masking fluid or forget to simply save the white areas while painting. Or perhaps you didn't realize you needed just a little touch of white to highlight something to make it pop.
Using a little bit of white titanium gouache to add a highlight can save the day.
Yes, white paint is often a no no in traditional transparent watercolors. There are rules in watercolor and then there are rules to be broken now and then.