The colour between red and blue. Historically the costliest dye, which is why it came to mean sovereignty.
Latin purpura (purple dye, purple cloth), from Greek porphyra (the purple dye drawn from a shellfish); purple was named for a colour so costly to extract that owning it became a mark of sovereignty.
The colour between red and blue, and historically the costliest dye a person could wear. The root's cost is why the colour still reads as rank: it never stopped meaning what only a few could afford.