Isidore of Seville, Etymologies 6.11:

On Parchment:

The kings of Pergamum were the first to think up membrana (skins) when they were in need of paper. For this reason, the name of Pergamenae has been preserved by the tradition of posterity all the way to our time. They are also called membranes because they are taken from the limbs (membris) of animals. At first, they were of a yellowish color (that is, the color of saffron), but later, white membrana were discovered in Rome. It was discovered that this was inconvenient, since they easily grew dirty, and would harm the eyesight of the reader, since indeed, more experienced architects think that golden ceiling panels should not be placed in libraries and that floors should not be made from any marble other than that from Carystos, since the resplendence of gold weakens while the greenness of the Carystean marble restores the eyesight. For those who learn money-changing place myrtle cloths on the forms of the denarii, and the engravers of gems immediately look at the backs of beetles, than which nothing could be more green, and painters do the same thing in order to restore the labor of their vision by their viridity. Some of the membrana are shining white by nature. The yellow membranum is bicolor, because it is dyed by its maker in one part – that is, it is yellowed. Thus, Persius writes

and now the book and the membrane, bicolor after losing their hairs.

Purple ones are tinged with a purple color, in which liquid gold and silver might show forth in the letters.

DE PERGAMENIS. Pergameni reges cum carta indigerent, membrana primi excogitaverunt. Vnde et pergamenarum nomen hucusque tradente sibi posteritate servatum est. Haec et membrana dicuntur, quia ex membris pecudum detrahuntur. Fiebant autem primum coloris lutei, id est crocei, postea vero Romae candida membrana reperta sunt; quod apparuit inhabile esse, quod et facile sordescant, aciemque legentiuni laedant; cum peritiores architecti neque aurea lacunaria ponenda in bibliothecis putent neque pavimenta alia quam e Carysteo marmore, quod auri fulgor hebetat et Carystei viriditas reficiat oculos. Nam et qui nummulariam discunt, denariorum formis myrteos pannos subiciunt, et gemmarum sculptores scarabaeorum terga, quibus nihil est viridius, subinde respiciunt, et pictores [idem faciunt, ut laborem visus eorum viriditate recreent]. Membrana autem aut candida aut lutea aut purpurea sunt. Candida naturaliter existunt. Luteum membranum bicolor est, quod a confectore una tinguitur parte, id est crocatur. De quo Persius (3,10):

Iam liber et positis bicolor membrana capillis.

Purpurea vero inficiuntur colore purpureo, in quibus aurum et argentum liquescens patescat in litteris.