Your title is misleading as it was not the Romans who used to do it. It was the phoenicians who lived in Tyre and actually sold it to the Romans.
More like Phoenicians and then Carthaginians did this, and Rome took up the trade when they came home from a day o' butchering. Also the kings of Rome were long dead at that point - wealthy Roman senators used the colour though and after Rome fell, the colour's significance was remembered and kept for nobles and royalty alike.
Deep purple was extremely hard to achieve considering they had to extract the smoke on the water.
Well...that's initially. But Tyre and the levant fell to Rome's Pompey late in the Republic. So...technically the Romans did used to do it.
There is a similar story about a red dye that was derived from a Mexican insect. For 3 centuries Spain had a monopoly on the dye and the secret of it's origin, which also was associated with royalty.
Because that's not quite how it works.
You might be thinking of additive color mixing, which is the model used for light (e.g. "red light" + "blue light" = vibrant "purple/magenta light"). Light combines towards white.
However, for mixing paint/dye/pigments, you need the subtractive model. Paint combines towards black. If you mix red and blue dye, you get some "muddy" violet/purple which might have looked quite different from the Roman's Tyrian purple. Plus, blue dye wasn't cheap either.
Edit: looks like the Romans actually did this! "The chemical composition of the dye from the murex is close to that of the dye from indigo, and indigo was sometimes used to make a counterfeit Tyrian purple, a crime which was severely punished. What seems to have mattered about Tyrian purple was not its color, but its luster, richness, its resistance to weather and light, and its high price." (source)
Carthage was founded by the Phoenicians (originally the same peoples), but might not exactly be a colony.
First of all, Plato was several centuries before the Romans. Second of all, it's quite likely that the first people who found that two colors can be mixed to get a third was someone who worked with colors for a living, not Plato.
Most likely, mixing the red and blue pigments available might have given an off-purple or even muddy color (because chemically, colors do not mix like they do optically). Or the mollusk shell based color was simply much more vibrant.
Deep red, deep purple, deep blue, true white and proper black were all really difficult colours to get.
In related news from the field of exclusive colours, british sculptor Anish Kapoor has recently patented the blackest man-made black, Vanta Black, and there is an ongoing shit-show between him and some guy who made his pinkest pink available to everyone but Kapoor in response.
Moreover, the word Phoenician is a word the Greeks used to describe the purple product: the Phoenicians never called themselves Phoenicians.
Just think if this naming method was now in use: the saudis would be called oilers, Italians silk, and the Germans autos.
Its not really a cool fact, was just pointing out that OP wasnt exactly justified in his criticism.
Why didn't they just mix blue and red pigments?
Ah, technically correct; the best kind of correct!
You mean... OP is a big, fat Phoeny?
It's a line from futurama.
So "Tyrian" is purple? Would that make Tyrion Lannister a "purple" lannister aka a Targaeryn?
It is so hard to get it out because they are shellfish
It's like, how much more black could this be?
And the answer is none, none more black.
Always trips me out that red light plus green light = yellow light.
It wasn't just any purple. It had a pearlescent coat that shone between crimson and deep purple. In other words it was Super Pimp and more dope than pleb purple.
All right... all right... but apart from better sanitation and medicine and education and irrigation and public health and roads and a freshwater system and baths and public order... what have the Romans done for us?
No, the answer is always Wesley Snipes.
Ceterum censeo Carthaginem esse delendam.
sure, a group not called Phoenician.
also, calling them an ethnic group might not be correct. they were a merging of many groups.
Also the mixing could have been considered a "poor mans purple" and royalty wouldn't want it.