These are three colors that are directly adjacent to each other on the color wheel. They relate to each other because they each contain one common color.
They are frequently used for a special feel in a painting and work very well together, but often give the feeling like something is missing.
To complete the feeling of balance in the painting, the complement of the dominant color can be used as a small accent.
If you don't want to use the complement, you can enrich the colors and add variety by
* Using white to lighten any of the colors, or
* By using the complements of each color to darken or gray them down.
This will add variety to the painting and still maintain the effect of restricting yourself to the analogous colors.
This is an example of green, blue-green, and blue being used together. They are next to each other on the color wheel with blue as the common primary color. It is blue because green is made from blue and yellow and blue-green is from the resulting green plus more blue.
To me, it gives the feeling of the sea.
This is the mixture of yellow, yellow-green and green. The common primary color is yellow.
The dominant color, however, is yellow-green since it has some of both colors in it, so your choice of a complementary color would be red-violet
This third example is of blue-violet, violet, and red-violet. In this case, the common color is a secondary color, violet. It is also the dominant color so the complement of choice would be yellow.
This last example is of red-orange, orange, and yellow-orange.
What do you think the common color is?
What is the dominant color?
What would be the complement?
For the answer, hover over the picture.