In Pesil Techeiles the Sixth Point (פתיל תכלת פרט הששי) the Radzyner Rebbe cites this Gemara as one of his proofs to the opinion of Rambam that one string is half techeiles and half white. The proof is that the Gemara asks what the Mishnah meant by "distinguish between techeiles and lavan", and rejects the possibility that it is referring to the ability to distinguish between a clump of blue wool and a clump of white wool, because that can be done even at night. The Gemara then concludes that it refers to the ability to distinguish between the techeiles and lavan in the tzitzis (as per Tosafos based on the Gemara in Menachos, and the Yerushalmi Berachos, both cited by the Radzyner Rebbe). The Radzyner Rebbe argues that the Rambam's view is the only one that gets around the issue brought up by the Gemara, of being able to distinguish even at night. He says that as long as the techeiles and lavan are separate they can be distinguished even at night. It is only according to the Rambam where there is a point on one string where it goes from techeiles to lavan that one would not be able to distinguish it until morning, because due to the impossibility of perfection in dyeing, the techeiles and lavan are almost overlapping at that point. The zeman for Kerias Shema is when you can distinguish between the techeiles and lavan at the point where it switches.
See here (last paragraph).