The use of white in watercolor is controversial. Some teachers tell students never to use white.
Arguments Against Using White in Watercolor
Water is the best way to make light values.
In other media, such as acrylics, oils, and gouache, white is one of the most frequently-used paints because it’s crucial to mix light colors, but watercolor is kind of different in that it’s transparent, so the most common/traditional/“watercolor-y” way to make light-valued colors is to dilute with plain ol’ water. As a beginner, especially coming from another style of painting, you may assume you need to use a lot of white paint in every painting, but actually many watercolor artists never use white, or use it only rarely, for specific effects.
White can make mixes appear chalky or muddy.
The opaque pastels you get from mixing white into other watercolor paints can have an “un-watercolory” look, more similar to gouache. Opaque colors like white turns mixes opaque, so you don’t get the transparent luminosity that is usually characteristic of watercolor.
Preserving your whites is a better way to create light-colored foreground objects.
Let’s say you want a white object in the foreground of your painting – such as a cloud or moon in front of a blue sky. In most painting styles, e.g. gouache and acrylic, you’d probably paint the sky first and then, when it is dry, paint the cloud or moon on top with white paint. However, the more traditional watercolor way of doing this is to preserve your whites – to protect them with resist (e.g. masking fluid or masking tape), or negative-paint around the white shape.
The reason for this is that watercolor is transparent, so light-over-dark doesn’t really work. Even when you have paints considered ‘opaque’, lights painted over darks in watercolor never end up looking as light as plain paper or diluted glazes.
Arguments For Using White in Watercolor
Sometimes you want the ‘chalky’ pastels mixed from white.
Opaque pastel mixes simply look different from transparent, diluted tints. It’s not either/or; sometimes you want one, and sometimes another. Opaque mixes are especially useful for solid or heavy-looking subjects, such as rocks, mountains, and stormclouds. There are also just certain shades of pastel pink or sky blue that you can only get from a mix with white.
Stars!
A starry sky is an example of a scene with white in the foreground that is impractical to paint by preserving whites or masking. Splattering white paint on a black background is a much more effective technique. (Technically, you can splatter masking fluid, but it’s much more annoying to clean up, and it can be a pain to remove it all from the painting.)
My Experience
The very first time I tried watercolor – from a set that included white – I assumed you made light colors with white, and my color mixes came out all wrong: insipid, blotchy pastels where I’d wanted luminous, bold colors. I wasn’t satisfied with them, and didn’t understand why I wasn’t getting that transparent watercolor look. I wish someone had taken me aside sooner and told me not to use white. So I think it’s absolutely appropriate for teachers to be super-clear with beginners that water is the main way of making light colors in watercolor.
But I’ve seen experienced artists go way too far in declaring a ban on white, saying things like “never use white” or “white is not used in watercolor.” White is incredibly useful for some situations! My main use case is stars, as noted above, but I also want to make a plug for using white to mix colors.
When I took a spring flowers class with Barbara Luel, I was fascinated with the unique pastel pinks that could be made with white and various reds. Before I (re-)learned about mixes with white, I was tempted by premixed pastel colors, such as Holbein’s Shell Pink; they’re nice and all, but it’s nothing you can’t get from a mix with white!
Currently, I keep Holbein Titanium White gouache on hand – not just because it’s one of the most important gouache colors, but for adding extras like stars to watercolor paintings done at home. Gouache functions better than watercolor for cover-up methods and can also be used as a mixer. However, it doesn’t rewet as well from dry, so it’s best when used from the tube.
For travel/plein air, I like to have Holbein Titanium White watercolor in my spring and winter palettes. I find it seasonally useful for soft snowy scenes and spring pastel color mixes. I find less use for it in fall, when warm colors dominator, or summer, when harsh-edged high-contrast shadows look best with wet-on-dry negative painting methods.
My actual advice
Water should be your default choice for making light colors in watercolor. However, white is an interesting and useful palette option. The types of light colors you make from water and the type you make from mixing white are different, and by getting to know both, you can choose the technique you want for the effect you want.