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Why is indigo separate from blue?

Why is indigo separate from blue?

Indigo is a deep and rich color that lies between blue and violet on the visible spectrum. While indigo is similar to blue, it has historically been categorized as its own distinct color. There are several reasons why indigo is considered separate from blue:

The history of indigo as a dye

One of the main reasons indigo is distinguished from blue is its history as a dye. Indigo dye has been used for centuries to color fabrics a deep blue-purple shade.

Indigo dye originally came from the indigo plant, which is native to Asia and India. In Ancient times, indigo dye was laboriously extracted from the leaves of the indigo plant. This process produced a distinctive blue-purple pigment that could strongly bind to fabrics.

Other blue dyes, like those made from woad or elderberries, tended to fade quickly. But fabrics dyed with indigo maintained a deep, inky color through washing and exposure to sunlight.

As a result, indigo became prized as a dye and luxury trade good. The distinctive indigo shade was seen as special and set apart from standard blues produced by other dyes.

The physics of indigo

Physics can also explain what makes indigo unique from pure blue. Indigo sits at a slightly longer wavelength on the visible light spectrum than blue.

The wavelength of indigo light is approximately 445–420 nanometers, while blue light is around 492–455 nm. This means indigo light oscillates at a slightly lower frequency than blue light.

This small difference in wavelength results in indigo reflecting slightly less red light than pure blue. The reduction in red light absorption shifts the color toward the violet end of the spectrum.

So while indigo appears similar to blue, its longer wavelength scientifically separates it as a distinct shade.

Indigo in art history

Throughout history, artists have recognized indigo as a distinct color from blue and created specific pigments to recreate its unique hue.

In Medieval and Renaissance eras, a pigment called ultramarine was highly valued for providing the rich indigo tones. Ultramarine was laboriously produced by grinding lapis lazuli gemstones. The high cost of ultramarine meant it was usually reserved for depicting the Virgin Mary’s robes.

Later, synthetically produced indigo pigments emerged, such as Prussian blue in the 18th century. This allowed artists to freely apply indigo hues without the limited supply of ultramarine. Impressionists like Van Gogh used indigo pigments to create signature vibrant pieces.

Artists have continued to recognize indigo as a vibrant and emotionally evocative shade between blue and violet. The continuation of indigo pigments and dyes signifies its enduring distinction from pure blue.

Indigo’s cultural symbolism

Beyond physics and craft, indigo also carries unique cultural symbolism that sets it apart from blue.

In many cultures and spiritual traditions, indigo is tied to wisdom, intuition, and mystical knowledge. This stems from its position between blue and violet on the color spectrum.

Blue is often associated with openness, peace, and calm. Violet meanwhile represents mysticism and spirituality. Indigo, falling between the two, takes on associations with inner focus, contemplation, and perception.

Hinduism views indigo as representing the third-eye chakra, the center of wisdom and intuition. Many Aboriginal cultures use indigo in face paint and ceremonies to represent metaphysical insight.

So while chemically similar to blue, indigo’s long ties to meditation and introspection lend it additional cultural distinction.

Modern classification systems

Most modern color classification systems recognize indigo as a distinct category from blue:

Classification System Indigo Definition
Munsell Color System (1905) Separates indigo as hue VS on the color wheel between blue and purple
Natural Color System (1940s) Places indigo in a separate perceptually uniform color space from blue
Pantone (1960s) Names a specific indigo pigment separate from blue
CIE 1931 Color Space Plots indigo at a longer wavelength than blue light
HTML/CSS Colors (1987) Includes named indigo value separate from blue options

As these color systems were formally developed, each recognized indigo’s perceptual distinction from blue and categorized it uniquely.

Distinction in language

The very fact that nearly all languages have distinct words for ‘indigo’ and ‘blue’ demonstrates their perceived difference.

Across cultures, indigo and blue are described as separate hues with different names. Some examples include:

Language Word for Indigo Word for Blue
English Indigo Blue
Spanish Índigo Azul
French Indigo Bleu
German Indigo Blau
Italian Indaco Blu

If indigo was perceived as the same color as blue, it’s likely most languages would not have distinct words referring to it.

Indigo’s sangria history

Notably, Newton originally named the color ‘indigo’ in his color wheel experiments in the 1600s. When initially plotted, the visible spectrum displayed seven distinct color categories.

While Newton could have lumped indigo with blue, he saw it as deserving differentiation. His recognition of indigo led to its adoption as the sixth distinct color of the rainbow between blue and violet.

Distinction in optics

Modern display technology relies on the same three types of color-sensing cones in our eyes. Red, green, and blue pixels combine to create the full visible color spectrum.

Notably, many displays use a technology called “RBG indigo” that includes a slightly longer wavelength blue specifically meant to recreate indigo hues.

This demonstrates that reproducing the indigo color range requires technology distinct from standard blue light emission. Engineers explicitly recognize and accommodate the difference between blue and indigo in color displays.

The indigo bunting

Indigo bunting There is even a species of bird named for its distinctive indigo plumage: the indigo bunting. This small songbird displays a brilliant indigo color during mating season. This unique shade inspired its common name and species Latin name, passerina cyanea, with cyanea meaning “blue.”

While referred to as “blue,” the indigo bunting’s plumage color is distinctly different from other blue birds due to its shorter wavelength. Ornithologists recognize the uniqueness of the indigo bunting’s coloration.


While indigo sits next to blue on the visible spectrum, it has developed a distinct identity through history, culture, physics, and language. Indigo’s connections with dyes, pigments, spirituality, and optics have firmly established its perceptual separation from blue for centuries across societies. Modern color science and technology continue to categorize indigo distinctly as we understand how our eyes and minds process color. So while closely related, indigo ultimately deserves recognition as a color in its own right, unique from blue.