Hi Keeping Alert,
I do not understand why you have not found my explanation to be satisfactory.
The word kokkinos, in the Greek, is the PRIMARY COLOR OF RED, to this very day.
See Greek-English dictionary.http://www.kypros.org/cgi-bin/lexicon
And, as I have shown, PORPHYRA is not exactly purple. It's purple-red.
Tyrian purple (Greek, πορφύρα, porphyra, Latin: purpura), also known as royal purple, imperial purple or imperial dye, is a purple-red natural dye, which is extracted from sea snails, and which was possibly first produced by the ancient Phoenicians.
So, basically, it's as if you, for example, were describing a car as being red; and another person, describing the same car, decided to be more specific, and described the same car, as burgundy.
Here is what comes up when you do a search to see what color porphyra is:http://www.colourlovers.com/color/B95D7B/Porphyra