fabric colors that were “always” available to common man were shades of everything from red to blue, white, black, brown and grays. purple was also always available, but extremely expensive.
enter coal tar era of chemistry (starting 1840s or so): aniline and later diazo dyes made fabric in all possible colors not only available, but cheap. yes initially they sucked, they ran, they weren’t resistant to anything, they will give you ballsack cancer, but they were a thing. for a short time, purple fabric was a choice of the extravagant and the futuristic, and then people just stopped paying attention as it became more common
blue LEDs were sort of also used as a futuristic aesthetic choice, just after they appeared, but before these things became common, and now blue LEDs are just everywhere
there’s indigo and another plant that grows in europe and also makes indigo but less, so you can just farm this thing, unlike purple dye that requires tons of work, and depending on period it was used by commoners (before 1200 or so, in western europe)
it’s a bit funny to look at this today, but woad (that euro indigo) trade was a big deal, it got protected by tariffs and blockades and diplomacy, and all for nothing, ultimately both woad and indigo farming was completely destroyed by synthetic indigo production. indigo wasn’t first/easiest dye to make, but it’s far from the most complex thing you can cook, even in 1900s. prussian blue is much cheaper than synthetic indigo anyway