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Because we don't know the dye that was used 3000 years ago, today most omit the blue threadWere I party to that contract, I (being aware of non-english distinctions) would wear any azure/голубой (sky, not ocean, blue), for:
a) a colour is specified, neither dye nor pigment. (pedantically, it need not even be a reflective colour, presumably something emissive, eg blue EL wire, would be equally acceptable)
b) 15:39 says the colour is meant for the eyes of the wearer, to remind them, so in this case the proper interpretation of "blue" is not even whatever that colour may have been 3'000 years ago, but whatever suggests "tekhelet" to the garment's wearer, today.
(in particular, a tritanope ought to be able to wear suitable grey threads)
Caveat: I have no hebrew, and english has lost its second person singular. Is the second person in the original singular or plural? In the latter case, (b) would need to be amended to match a community, not a personal, interpretation, and the tritanope should then be less free and more frum.
Edit: I think I see https://en.wiktionary.org/wiki/אתם#Hebrew in there, so it needs to be a community blue. Sucks to be a tritanope, but presumably english monoglots (who don't make the sky/ocean distinction in their basic colours; compare https://en.wikipedia.org/wiki/Color_term#Color-term_hierarch... ) would be fine with any blue.