Tynian Purple was the colour of royalty due to the cost of manufacturing the dye. Back in the day, purple dye was made by crushing up whelks and exposing the resulting goo to sunlight. A small portion of said goo will slowly change colour as it ferments, eventually turning a deep purple to royal red.
Now, for anyone that doesn't know, whelks are not exactly the largest creatures; in fact, as mollusks go, they tend to be on the extremely small side. With that in mind, simply dying the trim on Ceasars toga would take literally thousands of the critters, all of which have to be harvested, then crushed, separated, and finally mixed into a proper dye, by hand.
Now for the bigger question - why the hell do I know this?