Xanax comes in a variety of doses that are distinguished by their color. A Yellow Xanax Bar, like a white or blue Xanax bar, contains 2mg of active Alprazolam and has the imprint of R039 on one side of the drug. The various manufacturing companies cause the color difference. Yellow Xanax Bars, also known as "Yellow School Bus" or "Yellow Bus." This is a common name among drug users and sellers who buy non-prescription Xanax. Yellow Xanax Bars are prescribed to patients with moderate to severe anxiety and panic attacks due to their higher dosage strength.