출애굽기 29:29의 Halakhah

Yet it seems that since the Torah commanded us to use Techelet in the making of the Tabernacle and the Priestly Garments, and we have established in tractate Zevachim (18b) that for that Mitzvah we need that (the materials used for the Tabernacle) should be new and if they are defaced or filthy they are disqualified, see there. If so, evidently, if the likeness of their dyed state is faded they would be considered defaced and would be disqualified. And therefore, necessarily, the commandment of the Torah regarding the Techelet for the Priestly Garments was that it should sustain its beauty and not change, for we can not say that indeed for the Techelet of the Priestly Garments it was not necessary that its beauty be sustained and not change. And indeed, as of when the likeness of the dye fades it becomes disqualified and new garments would be made, for this is not the case. For the Torah said (Exodus 29) "And the Holy Garments of Aaron shall be for his sons after him." And indeed if the dye is not one that sustains its beauty, and it changes in and of itself, and fades, the fading would occur immediately from the beginning, and it would continue to fade. And indeed, we certainly need that the Techelet dye of the Priestly Garments be of a dye that sustains its beauty and does not change. And therefore we would say that wherever the Torah requires Techelet, it would need to sustain its beauty and not change. And therefore our sages, of blessed memory, required the Hillazon for the Techelet.