The opacity of watercolor provides opportunities available in no other type of paint. Because watercolor is meant to be put down in transparent layers (or washes), there are multiple methods for creating and mixing colors. And, of course, each method will give you slightly different results.
Because watercolor is transparent, you can layer unmixed colors on the paper to create a different color. This is known as optic mixing. For our example, I began by laying the Prussian Blue down first. I let this dry fully and then added the Alizarin Crimson on top. This may seem backwards since the blue is lighter than the red. But I prefer to put the lighter color down first because the darker color is more likely to smudge if a brush is dragged across it than the lighter color is. This is simply a result of the darker color leaving more pigment on the paper.
It’s very important, if you are layering the colors separately, to make sure that the bottom layer is 100% dry before adding to it. If the first color is even the slightest bit wet, adding the second color will create all sorts of staining and partial mixing and the end result will be a mess.
As you can see in the examples, even though the exact same colors are used in both, the results are slightly different. When the colors are mixed in advance, it tends to produce a more homogenous feel. When the colors are layered separately, they retain more of their individual characteristics.
Though this example only deals with the mixing to two colors, there is no reason that you can’t layer more to create your desired color on the paper. But be aware that the more pigment you have on the paper, the more likely you are to smudge it when you drag your brush across it.