In her book on Spanish cooking, Jenny Chandler writes that a cook should prefer the gray bacalao to the yellow fillets which she seems to imply are of lesser quality, the supermarket version of the authentic bacalao found in market stalls. What's the difference?
According to the Norwegian Cod lobby, a grey-blue cast indicates that the cod was caught in the deep sea, as opposed to cod caught in shallower, warmer waters. Presumably the deep-sea cod tastes better. Based on my own experience, salt cod which is too old or poorly stored will also turn yellowish. For example, the supermarket cod in the little wooden box, which is insufficiently dried and salted, is often yellowish with ageing. Again, based on my experience, this cod is noticeably inferior to the cod I used to get in the Sao Paulo Mercado Municipal, which was snow-white in color.
That said, if the yellowish stuff is all you can get, it's better than no salt cod.