Rambam in Peirush HaMishnah writes that we can’t make tekhelet because of color uncertainties, contradictory to what he writes in Hil. Tzitzit. Using modern science we are able to compare it to indigo. My question: is there only one color that can be produced from the indigo plant, or is it possible to adjust it to darker/lighter hue by adding or subtracting amounts of indigo?
First of all, I believe it is fair to say that in a case of contradiction that you mention, we use the Rambam’s Mishnah Torah (Hilchot Tzitzit) for halachic decisions. To the best of my knowledge the indigo plants produce only one color – indigo. However this does not mean that we can’t produce different SHADES, it is just a matter of dilution. What is important here is that the exact HUE be correct. Technically speaking, the hue is measured by the absorption spectrum of a particular object, regardless of shade. The indigo dye produced from the plants is precisely the same hue (has the exact same absorption spectrum) as the dye produced from the Murex snail. (By the way, this is one of the strongest points in favor of the Murex trunculus, since according to the Gemara in Baba Metzia 61 – only God can tell the difference between strings dyed with the plant (Kela Ilan) and strings dyed with the hillazon.) – Mois Navon.