Purple is known as a 'royal' colour because it was the hardest dye to extract and it came from sea snails, so only the royals could afford it.
The Phoenicians’ “Tyrian purple” came from a species of sea snail now known as Bolinus brandaris, and it was so exceedingly rare that it became worth its weight in gold.
To harvest it, dye-makers had to crack open the snail’s shell, extract a purple-producing mucus and expose it to sunlight for a precise amount of time.
It took as many as 250,000 mollusks to yield just one ounce of usable dye, but the result was a vibrant and long-lasting shade of purple.
Source: history.com
