Indigo (or spectral indigo) is the color on the spectrum between 440 and 420 nanometers in wavelength, placing it between blue and violet. Indigo and violet are different from purple (as defined by chromaticians color scientists) which cannot be seen on the electromagnetic spectrum and which can be achieved using additive primary colors by mixing blue light
with a red light.
If you see Sigma Aldrich Co , you can find 11 products for indigo and samereference material - one from its natural indigo - sub number / code ( R309494-1EA ).
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