Indigo is a secondary colour in the spectrum between blue and violet. ROY G. BIV is an acronym for the colours of the rainbow, the "I" standing for the colour Indigo. In traditional depictions of rainbows, indigo is often left out because it is so close to blue.
The primary colours that make up indigo are red and blue. Red and blue also can be mixed to make violet when used in equal parts. To make indigo, blue has to be the dominant colour in the equation. The mathematical equation to produce indigo would be to mix one-third red and two-thirds blue. Red is a strong dominant colour, so you would need to add a lot of blue depending on how bright your red paint is.
The equation between red and blue can be tweaked as needed or to get a different hue of indigo. A "hue" is how much a secondary colour leans toward one of the primary colours. The primary colours are red, blue and yellow. The secondary colours are all composed of different ratios of the primary colours.
Shades, Tints and Tones
If you wanted a darker shade of indigo, you would need more blue, or you could add a little black. White can be added to achieve a lighter tint. You can also add both black and white to tone down indigo if you don't want it to be too bright.
Indigo is used in art mainly when painting night skies or landscapes. The colour indigo is somewhat rare to find in nature. Synthetic indigo is used in differing amounts to dye denim for blue jeans and other kinds of fabric. In colour therapy indigo is used to help treat an overactive thyroid and is used in meditation to calm and relieve stress.
Use a Prism
Another way to make indigo visible is to use a prism to shine white light through it. White light is made up of all the colours of the rainbow. Although it can be muted and blend in seamlessly with blue and violet, it is always there.