Reading a wikipedia page about prostitution (it is in spanish; I haven't checked if the english says the same) it is said that prostitues wore purple dresses.
Was this generally true?
I would have to say no, but there is quite a bit of primary literature about the attire of prostitutes in ancient Rome.
The more scholarly debate on Roman prostitutes' attire is the dispute as to whether prostitutes in Rome wore togas as a distinctive garment, and much of the contemporary texts distinguishes only between matrona (a "respectible" woman) togata (ostensibly a prostitute or adulteress).
A good example, and the closest I can find to anything approaching a reference to purple is Martial's Epigrams 2.39, where he writes:
Most evidence points toward a wide range of attire worn by prostitutes, driven more by market demand and social status than anything else. Roman prostitutes were almost always slaves, and as such, that would have driven what they wore as much as anything else. Thomas McGinn writes in Prostitution, Sexuality, and the Law in Ancient Rome that,
This is also evident in the analysis by Kelly Olson in the essay entitled Matrona and Whore: Clothing and Definition in Roman Antiquity, in the anthology Prostitutes and Courtesans in the Ancient World:
Much, much, more likely was the cultural association with certain forms and modes of dress that were correlated with prostitution - something akin to "you'd know it if you saw it". To say that Roman prostitutes wore purple (or togas, or anything else for that matter) is akin to saying that American prostitutes wear fish-net stockings. More metaphor than anything with a basis in fact. Who knows, maybe in the year 4014 somebody will post a question on whatever passes for History SE then asking if ancient American prostitutes worn them.
The colour and clothes worn in Ancient Rome directly reflected your social status. Purple on your clothing indicated that you were high up the social ladder:
I was once told that a full purple toga was reserved for the emperor, although I can't confirm this at the moment and Wiki says that wealthy women wore it (must depend on the period).
The choice of purple for the upper-class was simply because it was the most expensive.
If some of the average prostitutes did actually wear purple it must have been the cheap knock-off. Perhaps similarly to how today some prostitutes may dress up in "cheapy" clothing that tries to imitate the expensive stuff. The vast majority were slaves though so the clothing they wore would have been a decision of the pimp. Maybe they noticed that dressing them as upper class ladies increased sales.
There were also what you could perhaps call 'luxury' upper-class prostitutes in the ancient world. Although they wouldn't have considered themselves as prostitutes, the principle of exchanging services for material gain from respectable individuals was there. These women could have afforded nice purple clothes.
However I couldn't find indication anywhere that it was common for prostitutes to wear purple, so the answer is likely no, but maybe in some cases.
Source: Clothing in ancient Rome
Perhaps some (of the higher end ones) did, but as a general rule this seems exceedingly unlikely.
What is almost certainly meant by a "purple toga" in Roman times is the Toga picta. I can't find any source that even references a female wearing one.
Of course there's a reason purple was so special: It was hideously expensive. The dye used, Tyrian Purple is extracted in a laborious and touchy process from tiny glands in a specific species of coastal sea snail.
A chemist attempting this process a few years back reportedly required 12,000 snails to get enough dye for a handkerchief.
So if any woman did wear a toga picta, she would have had to have been very well off indeed. Its possible a very high end courtesan, or a favored mistress of an extremely wealthy senator might be able to come up with one. However, there is simply no way a common street prostitute could wear one.