Figure 12. Chromatic aberration from a thin lens. The refractive index varies with wavelength, causing each wavelength to come to a focus at a different distance. Here only three wavelengths are represented, coloured blue, green and red. Blue comes into focus first, then green, then red last. The image is a mixture of the blur discs of these three wavelengths. In reality, with more wavelengths, the blur would look like concentric rainbows.
You can turn off some of the wavelengths using the checkboxes to see what happens to the wavelengths individually.