Here is an interesting description: "The miniature in Paris. gr. 2243 offers additional information which no doubt applies to all hospital personnel, not just those assigned to the outpatient clinics. In the illustration each person on the hospital staff has a distinctive gown and conical hat. The physician wears a blue-green gown, a color traditionally assigned to doctors since late antiquity. Over the gown he has a red mantle which matches his red half-boots. his conical hat is violet. The hypourgos [male medical assistant] wears a violet tunic, red half-boots, and a pale yellow conical hat with a red vertical stripe. The pharmacist wears a variegated red and blue tunic, yellow half-boots, and a red conical hat with a vertical whipe stripe. The colors of the tunics, boots, and hats clearly denote rank and function [...] From other sources describing medical men, however, one can verify the significance of only the blue gown as the prescribed uniform of physicians."
Text source: Miller, Timothy."The Birth of the Hospital in the Byzantine Empire". Baltimore and London: The John Hopkins University Press, 1997. Pp.155-6
Another, later image shows a different hat, but the blue robe stays the same:
Bologna, Biblioteca Universitaria gr. 3632, folio 134v. "Physician Isaac holds Urine flask", 15th c.
(text source: http://dc.lib.unc.edu/cdm4/item_viewer.php?CISOROOT=/mackinney&CISOPTR=2465&CISOBOX=1&REC=4 )
Another 15th century illustration (showing the difference of dress between doctor and assistant, but still none of the hats in the first illustration) is this:
Bologna, Biblioteca Universitaria gr. 3632, folio 255v. Serapion with medicine jar (left); assistant with mortar and pestle (center); Praxagoras (right); 15th c.
The same site provides (inter alia) two depictions of physicians (Oribasius and Philo Judeaus) wearing large yellow over-robes. Oribasius has a red undertunic while Philo wears blue, and, despite the color difference, their outfits are basically identical. Since both of these physicians were alive in late antiquity, this may be an anachronistic depiction, especially since the images I posted here all illustrate relative contemporaries (at least they all fall in the mid-late Byzantine period).