If you come to watercolor painting from oils or acrylics, you may find yourself wondering where the tube (or pan) of white paint is. The answer is: In watercolor there isn't one. The white in watercolor painting is the paper. You "make" white by having the paint thin so the white of the paper shows through. Be aware that you will want to take the color of your paper into account when planning your painting and carefully think through where your white will be in the final design.
Out in the real world, white is not really pure white but can be a warm white that reflects yellows or a cool white that contains more blue. (And one trip to the hardware store with the thought of painting a room "just white" will present you with more color choices than you could ever imagine for such a seemingly straightforward task!) To determine if you should paint something warm or cool, look at the object in diffused daylight to see what colors reflect in it. If the light is too bright, it will create sharp shadows and "blown out" areas; you need to see a range of tones. Please note that even "daylight" bulbs that purport to be balanced can still cast a yellow hue on your subject.
Any white in a surface will be affected by the light glowing on it, such as showing a warm yellow cast from an incandescent bulb, and any shadows falling on it. Shadows will be brought out in a painting with purple, blue, and gray or even light brown. They are a necessity if you have a white object against a white background or want to paint definition in white fabric, for example. What remains after the color is applied around the white area shows the white off through contrast and makes the remaining white turn into the highlight of the area.
You can create a gradient that includes the white paper through placing subtle washes (very diluted) next to the white area and progressing a little darker a bit at a time to have "shades" of white without having any white paint in the mix.
To "save" an area and keep it white so other colors don't bleed into it while you're painting, you can use the wax relief technique where a tiny bit wax from a white candle or white crayon drawn on the area will keep the spot the white and won't be visible in the final product.
If you're not a purist, not one of those who believe that only transparent water-based paints qualify as "watercolor," then get yourself a tube of white gouache, also known as opaque watercolor. Some artists also use white acrylic, but remember, as soon as acrylic paint has dried, you can't lift it off again, unlike watercolor paints and gouache, which remain water soluble.
Don't use gouache to mix your colors, though, as it'll only muddy your tints. Lighten your colors through washes. Use opaque white as a final highlight or to achieve an effect that would be difficult to do in any other way.
Please note that if you're submitting a painting to a watercolor competition, you will want to check whether the rules allow for white paint. Some do, and some don't.