<P> A spectrum obtained using a glass prism and a point source is a continuum of wavelengths without bands . The number of colours that the human eye is able to distinguish in a spectrum is in the order of 100 . Accordingly, the Munsell colour system (a 20th - century system for numerically describing colours, based on equal steps for human visual perception) distinguishes 100 hues . The apparent discreteness of main colours is an artefact of human perception and the exact number of main colours is a somewhat arbitrary choice . </P> <Table> <Tr> <Td> Red </Td> <Td> Orange </Td> <Td> Yellow </Td> <Td> Green </Td> <Td> Blue </Td> <Td> Indigo </Td> <Td> Violet </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> </Table> <Tr> <Td> Red </Td> <Td> Orange </Td> <Td> Yellow </Td> <Td> Green </Td> <Td> Blue </Td> <Td> Indigo </Td> <Td> Violet </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr>

What are the colors that are not in the rainbow