Why use a limited palette
Try painting a picture that contains both quinacridone rose and cadmium red, or French ultramarine and phthalo blue. The coloring achieved with such a color combination is not beautiful, it is disharmonious, like tritone in music.
In painting techniques other than watercolor, this is important for how colors harmonize with each other. In watercolor, the difference is even greater. This is because paints not only have a hue, they also have other properties that emerge especially in watercolor: Some colors granulate, others leave a hard edge. Some are transparent while others are opaque. These properties come into their own in watercolor, while they are more hidden in other techniques.
To achieve harmony in a painting, I choose to paint with few colors. I never use ready-mixed colors, always colors with only one priment. Why limit your choice of possible color mixes with ready-mixed colors. They may have their place unmixed, but it is not good to mix further with a ready-mixed color, there will be far too many different pigments that way. The result will be dirty and cloudy.
When two colors are mixed, the result is darker than the two colors used. When a third color is mixed in, it becomes even darker, and so on with the fourth and fifth pigments involved.
Say you are going to paint some olives, then you might use olive green. Olive green is not a pigment, it is just the name of a certain hue. You may use (like most watercolor painters in Sweden) Winsor & Newton, in which case you have used yellow ocher and phthalo green, these are the colors that this color mix contains. Now you want to make the green color a little darker and use the popular Payne’s gray for this, it consists of phthalo blue, lamp black and quinacridone rose. By mixing two colors, you have in fact mixed five different pigments. You risk getting dirty colors and incoherent coloring that way. Most people do this, I have noticed, they use popular ready-mixed colors regardless of what they actually contain.
It is to achieve harmonizing colors and controlled mixtures that one uses a few colors in the same painting. In the past, when artists only had access to a few pigments, this was not something that was considered. During the 19th century revolution in chemistry, which i.a. resulted in a lot of new colors, this became more noticeable. Today, when artists have access to hundreds of different pigments and all sorts of ready-mixed colors, this is something worth thinking about.
Anders Zorn is famous for his limited palette: Yellow ocher, ivory black, vermilion and titanium white. He used other colors as well, but these four are his base and many of his paintings are painted with only these. I myself have more colors than four, but never in the same painting, so even though I have many different colors, I only use a few at a time. For the next painting, I choose a few others.
How I reason about color choices, I will write more about in future blog posts.