Why did early doctors wear red robes?

Since medieval times, doctors, like bishops and cardinals, have been authorised to wear garments of brighter colours such as scarlet, purple or red. In many older universities, doctors have scarlet dress gowns or robes (sometimes called "festal robes") which are worn on special occasions (for example, at graduation ceremonies and on certain festivals of the Christian calendar), as well as black undress gowns which are worn on ordinary occasions. A third form of dress, now rarely seen except at Oxford, is the Convocation habit. This is a scarlet sleeveless garment worn over the black gown, with the sleeves of the gown pulled through the armholes. It is worn at meetings of Convocation or Congregation by those presenting candidates for degrees.