It probably is just and egg that is in the process of going bad. Just wait and see if it deflates or mold. I've had 3 of the 48 from this clutch go bad so far and they all three turned tannish/yellowish/brownish then deflated and molded. If its onl a couple i would say it has nothing to do with the medium but if they all start doing they it could be too wet or something of that sort. I always get upset when even just one egg goes bad because I imagine that it could have been the best looking one of the clutch and it kills me. Good luck with the rest of them.
Justin