There are 7 colors of the rainbow. You can remember them because they spell out ROY G BIV. The first letter of the colors red, orange, yellow, green, blue, violet and indigo.
Rainbows are commonly thought as having 7 colors. The colors in order are:
However, indigo and violet are close enough on the color wheel to represent the secondary color resulting from the combination of primary colours red and blue. An alternate answer is 6 for cases in which either indigo or violet is omitted or both are replaced with the term “purple”.
If you are referring to how many colors in terms of big numbers, it’s pretty much infinity, but a smaller infinity than how many colors there are. For example, black, white, brown, and pink are not in the rainbow.
A rainbow is an optical and meteorological phenomenon that is caused by reflection of light in water droplets in the Earth’s atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc.
Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
In a “primary rainbow”, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted while entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.
In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.
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. However, the human brain tends to divide them into a small number of primary colours. The apparent discreteness of primary colours is an artefact of the human brain. Newton originally (1672) divided the spectrum in five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven primary colours by analogy to the number of notes in a musical scale. The Munsell colour system removed orange and indigo again, and returned to five primary colours. The exact number of primary colours for humans is a somewhat arbitrary choice.
The colour pattern of a rainbow is different from a spectrum, and the colours are less saturated. There is spectral smearing in a rainbow due to the fact that for any particular wavelength, there is a distribution of exit angles, rather than a single unvarying angle. In addition, a rainbow is a blurred version of the bow obtained from a point source, because the disk diameter of the sun (0.5°) cannot be neglected compared to the width of a rainbow (2°). The number of colour bands of a rainbow may therefore be different from the number of bands in a spectrum, especially if the droplets are either large or small. Therefore, the number of colours of a rainbow is variable. If, however, the word rainbow is used inaccurately to mean spectrum, it is the number of primary colours in the spectrum.