Nic oxides will always form, and are strongly colored compounds. Even in trace amounts they will color a juice, and at room temp if the bottle has been opened, they cannot be avoided. The different colors come about because of the environment, which can make for a yellow to orange most often, or even a pink rose color. It does NOT mean the juice is "bad", since the oxides that form are not at all harmful, and they will color a juice even if 0.1% of the nic oxidizes. My unflavored VG nic juices will go yellow after a time at room temp. My vanilla DIY juices will often go rose colored after some time. They are still good juices.