Purple became the color of royalty because the dye was so expensive.
In the 16th century BCE, a precious treasure emerged from the Mediterranean Sea: Murex brandaris, a type of sea snail. During the Bronze Age in the Phoenician city of Tyre (modern-day Lebanon), the sea snails were used to make Tyrian purple dye, also known as “Royal” or “Imperial” purple. To create the dye, a yellow fluid was extracted from the snails and exposed to light until it turned a brilliant shade of purple. The fabric dye became fashionable across Phoenicia because it was more vibrant and longer-lasting than existing dyes. However, it was also much more expensive to make. It took up to 12,000 mollusks to produce just 1 gram of dye, and Murex snails became worth their weight in gold.
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Wearing purple was illegal in Elizabethan England unless you were royalty.
The purple hue has long been associated with the British monarchy. Even today, the coronation crown, St. Edward’s Crown, has a lavish purple velvet cap. But in Elizabethan England, you could be fined or even jailed just for wearing purple. In the 16th century, Queen Elizabeth I issued several sets of “sumptuary laws” designed to regulate expenditures for commoners, often by limiting or banning luxurious items. In 1574, purple was at the top of the list. According to the statute, only the monarch and their family members were allowed to wear purple silk. Plenty of other costly fabrics were also limited, including red shades of velvet, which were reserved for nobility, and gold, silver, or pearl embroidery.
https://historyfacts.com/world-history/fact/purple-color-of-royalty/
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