White in Watercolor
White in watercolor is usually considered to be the paper itself.
Watercolor is so transparent that pastel shades can only be achieved by diluting the paint heavily with water. Leaving white areas for highlights without using any additional tools is considered the highest level of skill, and only very few artists are able to do it well.
But what should we do if we are not yet skilled enough to achieve this? There are several ways:
Masking fluid - this is a rubbery substance that is applied to dry paper, and after it dries the entire area is painted over with a single wash. The fluid should be removed only when the paper is completely dry, otherwise, you won't get perfectly white highlights. The disadvantage of this method is that you have to wait for everything to dry before and after applying it. Also, the boundaries of the areas can be too sharp.
White gouache is a popular tool among many artists. Gouache is applied with a thin brush where needed, as a final touch.
Scratching off highlights with a knife. This method is slightly brutal in my opinion.
There is also an ancient method when there were no masking fluids around: wax candles. They were used to mark in advance the areas on the paper where white highlights should be left, and water would simply roll off them. But I wouldn't recommend practicing this method, as I'm not sure how the wax will affect the paper over time.
Practical tips:
Masking fluid can be applied with a pen or a ruling pen.
If you apply it with a brush, soap the brush beforehand, so the fluid won't stick to it.
"Sennelier" has the most liquid masking fluid.
Try removing the fluid with a cloth or a piece of lace, so you don't smear the paint with greasy fingers.
Summary
Use gouache or masking fluid to help yourself with the white spots if you have no choice, but try to learn how to manage without them.