A minor goddess in Greek mythology, Pasithea was one of the Graces, associated with relaxation and hallucinations. And being the wife of Hypnos, the god of sleep, would also make her the sister-in-law of Thanatos, the god of death. As well as related somehow (depending on your sources) to the various personifications of dreams and nightmares, including Morpheus after whom morphine is named.

All pretty appropriate for the drug, I reckon.

A fair enough question, given that they use the colours as a form of address. I guess it’s down to the reader, but FWIW in my mind it means yellow and it’s pronounced “yellow”. Though I do sort of imagine it as a quick and fluid, unaccented prefix to the names – unlike when they call each other “Citizen Red” or “Citizen Orange” or whatever more formally.