The color purple has long been associated with royalty because it was so expensive to produce the dye that only wealthy rulers could afford it. During the Victorian era, a chemist named Henry Perkins was able to create a synthetic purple compound to create the dye. Perkin's synthetic purple was able to be produced more efficiently, making it more affordable. Queen Victoria herself wore a purple dress created with Perkin's mauve to her eldest daughter's wedding, according to Jacqueline Banerjee, associate editor for The Victorian Web. Interestingly, the word porphyria comes from the Greek word porphura meaning purple. Perhaps Robert Browning chose to name his character Porphyria to represent that she was of higher social status than the narrator. The fact that Porphyria not only initiates sexual contact which would have been seen as unbecoming for a woman of this time but to initiate this contact with someone of lower social status would have been egregious. Porphyria's lover could have murdered her as a means to maintain this "forbidden love" between the two because of their different classes.
Banerjee, Jacqueline. “Sir William Henry Perkin and the Coal-Tar Colours.” The Victorian Web, 28 Aug. 2014, victorianweb.org/science/perkin.html.