Purple is a general term for the range of shades of color
occurring between red and blue. It is formed (in both
subtractive pigment and additive light combinations) by mixing
the primary colors red and blue in varying proportions, with
possibly a very small quantity of the third primary color (green
for light or yellow for pigment). There is disagreement over
exactly which shades can be described as purple, some people
preferring more precise terms such as magenta or heliotrope for
particular shades. A difference in retinal sensitivity to red
and blue light between individuals can cause further
disagreement.     Etymology and definitions  The word 'purple'
comes from the Old English word purpul which originates from the
Latin purpura. This in turn is derived from the Koine Greek
πορφύρα (porphyra), name of the dye manufactured in Classical
antiquity from the mucus-secretion of the hypobranchial gland of
a marine snail known as the Murex brandaris or the spiny dye-
murex.   Purple versus violet  Violet is a spe