Reference for Shevuot 12:1

ואם איתא של צמר דיהה הימנה מיבעי ליה

And if it is [as you say, that R'Akiba holds they are one above the other, i.e., Bahereth, then Se'eth], he should have said: White wool [i.e., Se'eth] is fainter than it? - That is so [R'Akiba really said Se'eth, and n Sid]. And so said R'Nathan: R'Akiba did not say: Sid is fainter than it, but white wool [i.e., Se'eth] is fain than it.

Jastrow