Your sales associate is incorrect. The fading has nothing to do with exposure to oxygen and not all pink gold fades. I have a few rose gold watches that are older than my love bracelets, and one is far Cartier, they have not faded a bit. They have all been exposed to air/oxygen, but never been exposed to water (not worn in showers or pools). I have also read about owners of rose gold watches who's owners complained that their "Everrose" Rose gold (which is a special 18Kt rose gold alloy paired with platinum that is supposed to resist fading) has had instances of fading after significant exposure to water. In the end, lots of our RG bracelets have or are fading...but from my own experience what what I've read its not due to oxygen. Cheers!
PS. My RG love is 9 years old and has fade to the point where by itself it looks almost yellow other than the screw motifs which have a hint of pink to them. However, when I put it next to my YG love, you can see the difference in color between the rose and yellow gold.