Watercolor or water-soluble pencils and crayons are a unique cross-over between drawing and painting. You draw with them as you would with any pencil or crayon, but then if you run a wet brush over your drawing, the color is dispersed and turns into a watercolor wash. They have the advantage of being easy to use, relatively cheap, and don't leave you with a mess to clean up.
Watercolor pencils are specifically manufactured with a binder that dissolves in water.
Water-soluble pencils are available in a wide range of colors, as well as plain graphite pencils. Colored watercolor pencils aren't graded like graphite pencils are (from 9B, the softest, to 9H, the hardest), but their softness does vary between brands so it may be worth buying a sample pencil from various brands to see which you prefer before you buy a set. The softer a watercolor pencil is, the easier it is to put color or pigment down on a paper.
Two variations on watercolor pencils available are woodless pencils (just the pencil 'lead' with a paper wrapper) and water-soluble crayons (like wax crayons, but they dissolve in water). Water-soluble crayons enable you to put down more pigment (or color) faster than a watercolor pencil, as they're softer and broader.
Watercolor pencils look the same as 'normal' pencils, but if you check the lettering stamped on them you'll see a little symbol to show they're water soluble, such as a water droplet or a small brush, or the word 'watercolor'. Of course, you can always just do a quick test on a scrap bit of paper to test.
Using a water-soluble pencil or crayon where you didn't intend to can result in disaster if you use to paint over the drawing or sketch, smudging it. So if you mix your types of pencils up, always check!
Using watercolor pencils is very similar to using a 'normal' pencil or color pencil. You hold them the same way, you sharpen the same way, and you can erase them.
It's when you add water into the equation that their uniqueness appears. There are different ways you can do this. For starters, you can do by painting with clean water over your drawing. But you can also lift paint off the pencil with a brush then apply it to your paper, wet the pencil then draw with it, or wet the support you're working on.
By 'painting' over watercolor pencil with a brush that's been loaded with clean water or a water brush, the pencil lines 'dissolve' into the watercolor paint. The intensity of the wash produced depends on the amount of pencil that had been applied to paper. The more pencil 'lead', the more intense the color.
Be selective in which areas you turn into washes to make the most of the unique properties of watercolor pencils if you turn every bit of watercolor pencil into a watercolor wash, you may as well have used watercolor paints to start with.
To load a brush with a particular color, treat the pencil tip in the same way you would a pan of watercolor: wet your brush, then use the brush tip to pick up the color from the watercolor pencil.
If you dampen your paper before you apply the watercolor pencil, you'll get softer, broader lines of color than if you draw on dry paper. Work carefully, and don't use pencils that are extremely sharp, so you don't damage the surface of the paper.
Another option is to wet the tip of the pencil or crayon before you use it. If you dip the tip of a watercolor pencil into some clean water or dampen the tip with a wet brush, then draw with it, you'll get lines of intense color. As the pencil dries out, the line will become lighter and thinner.
As with ordinary watercolor paint or pencil that is not water soluble, you can use as many layers as you wish. You just continue working. That said, too many colors and you risk creating a color that looks like mud rather than anything else.
The extent to which the colors mix depends on how hard you scrub with the brush at the pigment you've applied to the paper. If you go back and forth, back and forth, you'll dissolve all the pigment. If you just go lightly over the top, you will only dissolve the very top.
If you're working on a textured paper, you can use this property of water-soluble pencil or crayon to creature texture or a granulated effect as shown in the photo above.
When working wet on wet with watercolor pencils or water-soluble crayons, make a point of keeping the tips clean to ensure your colors don't get muddied. Wipe the tip on a damp cloth or scribble with it on a scrap bit of paper.
Scraping color off a watercolor pencil is a great way to create texture. Use a knife to scrape off tiny bits of pencil. Sprinkle these onto wet paper, or drop a bit of water on top of them, and watch the color spread out.
Don't get so seduced by the watercolor properties of watercolor pencils that you ignore the rich color and detail you get when using them 'dry', in the same way, you'd use ordinary colored pencils. Leave some of the pencils undisturbed, or apply fine detail with a dry pencil once the washes have dried.