Indigo, a color often listed as one of the seven hues of the rainbow, frequently sparks debate. You might encounter discussions online, even Facebook groups, questioning its validity as a distinct color in the spectrum. This article delves into the question: What Color Is Indigo, and why is its place in the rainbow sometimes disputed?
Sir Isaac Newton, in his study of optics, famously identified seven colors in the rainbow: red, orange, yellow, green, blue, indigo, and violet. Some historians suggest that Newton, influenced by the significance of seven notes in a musical octave, sought to find a similar structure in the visible spectrum. However, contemporary observers often argue that only six distinct colors are readily discernible in a rainbow.
Painter's color wheel showing primary (red, yellow, blue) and secondary (orange, green, violet) colors, used in arguments against indigo's inclusion in the rainbow.
One common argument for removing indigo from the rainbow spectrum stems from color theory, specifically using the painter’s color wheel. This perspective, sometimes seen in online groups, claims indigo is not a primary or secondary color, but a tertiary one. The color wheel typically presented in these arguments highlights red, yellow, and blue as primary colors, and orange, green, and violet as secondary colors. Since six rainbow colors align with primary or secondary colors on this wheel, and indigo does not, the argument is made that indigo is an unnecessary addition to the rainbow. However, this line of reasoning contains several fundamental flaws.
The primary oversight in this argument is the confusion between subtractive and additive color mixing. Additive color mixing, relevant to light and the spectrum, describes how light combines. The additive primary colors are red, green, and blue (RGB). When mixed, these create secondary colors like cyan, magenta, and yellow. Notably, orange, a recognized rainbow color, isn’t even a secondary color in the additive system. Given that the rainbow is formed from emitted light, additive color principles are more applicable when discussing its colors.
Subtractive color mixing, on the other hand, pertains to pigments and dyes. The optimal subtractive primaries are cyan, magenta, and yellow (CMY), yielding red, green, and blue as secondaries. The artist’s color wheel, often resembling the painter’s wheel mentioned earlier, traditionally uses red, yellow, and blue (RYB) as primaries. Confusing these systems leads to inaccurate conclusions about the colors present in a light spectrum like a rainbow.
Ultimately, the colors we perceive in the spectrum, including indigo, are not inherently “special” based on color wheel classifications. Orange, for example, is clearly a blend of red and yellow, and its inclusion in the rainbow isn’t questioned based on its “purity.” Therefore, arguments against indigo’s place in the spectrum based on color wheels or primary color status are not scientifically sound.
The more pertinent question is whether indigo is phenomenologically distinct in the visible spectrum. Many individuals find it challenging to differentiate indigo as a separate band between blue and violet. Some, like the author of the original article, find indigo and violet indistinguishable in the rainbow. This observation aligns with the sentiment of groups advocating for the removal of indigo, even if their reasoning based on color wheels is flawed.
The truly fascinating question then shifts to: why do we perceive distinct color bands at all in the rainbow or spectrum, whether six or seven, when the wavelengths of light transition smoothly and continuously? While the precise reasons for this discrete color perception are complex and potentially linked to how our brains categorize visual information, it moves beyond simple color wheel arguments and into the realm of perception and neuroscience. Understanding “what color is indigo” requires acknowledging both its spectral position and the subjective nature of color perception itself.
