Are there colors that do not occur naturally?

When you look at a rainbow, there are a limited number of colored stripes that we see. These represent the range of the colors that we can see due to a single frequency of light. There is, for example, no pink or brown. We can create a large range of colors by mixing colors of different frequencies. I believe the full range of colors can be achieved using the full range of possible RGB values, causing varying stimulation patterns to our eye cones. Are any of these colors ones that do not exist anywhere in nature?