Halakhah sur L’Exode 28:31

And futhermore it is understood from the words of our sages of blessed memory that the techelet regarding that which the Torah has enjoined us must retain its original beauty and not fade in color, this can be learned from the passage in Exodus 28 where it is written 'completely techelet'. And when this color fades it is not completely techelet. And as Rabbi Chanina the son of Rabbi Gamliel expounded regarding a blemish which makes it inadequate for use (Menachot 42b). And even though Rabbi Yohanon the son of Dahavai disagrees there and says that even the second appearance of the color of techelet is fit for use, that means only at the begining of the dyeing process, but once it has been dyed for the Mitzvah and then it fades and continues to get lighter in color so then certainly it is not considered to be completely techelet and it must retain its original beauty and not change. For if its color is not one that retains its original beauty and does change then certainly it is not fit for use as soon as it begins to fade from its original color and changes. Therefore when the Torah states completelytechelet then certainly it means that it must retain its original beauty and not change. For the techelet of the tzitzith is learned out from the techelet of the priestly garments, for all techelet mentioned in the Torah must retain its original beauty and not change. Therefore the sages of blessed memory needed the chilazon that was known to them, for the color of techelet from any other species does not retain its original beauty and fades.