I'm in an argument with a friend and could use some help. He claimed that prostitutes in the 16th century wore red skirts if they were young to "hide the blood" and green skirts if they were more experienced to "hide the grass stains". I personally think this belongs in r/badwomensanatomy and after some research the only identifying color I could find for middle age prostitutes was that Italy made them wear yellow.
Is there any accuracy to what he's saying?
The specifics of this code don't sound like any sumptuary law related to prostitutes that I've ever heard of, for sure, nor the occasional hints I've seen of what prostitutes chose to wear outside of (sometimes in contradiction of) laws. But more to the point, your friend is misunderstanding the concept and purpose of sumptuary laws altogether.
First, as you've already discovered, there were plenty of city and even parliamentary-level laws in the later Middle Ages into the early modern era that regulated people's dress according to social status. Sometimes this meant wealth or nobility versus non-noble background, sometimes an array of professions ranked according to social status, and sometimes particularly stigmatized groups like Jews or prostitutes would be singled out.
In terms of specifics, the laws usually mentioned with respect to prostitutes required, in various times and places:
Concerning your particular colors: James Brundage briefly mentions that prostitutes in Zurich, the law being passed in the 13th century, were required to wear red caps (a hat is associated with losing one's innocence, i.e. virginity; this is distinct from a nun's habit for obvious reasons). He also says that Augsburg at some point required prostitutes to wear green, but I haven't been able to back this up and am a little skeptical. Executioners, another stigmatized group, were required to add strips in the city livery colors (one of which was green) to their clothing; this sounds more reasonable for prostitutes (especially since executioners were sometimes responsible for running brothels) given the apparent difficulty (ergo expense) of achieving a true green dye in the Renaissance era.
But even setting aside specific laws, since it would be foolish of me to claim I had a list of every single one, your friend is misunderstanding the point of sumptuary laws. This is not to make things easy for potential customers. There were two basic principles generally in action. First, to protect the economic interests of the city or country at hand. Banning the non-elite (and sometimes even the elite) from wearing garments of foreign cloth or imported decoration discouraged the importing of said textiles and encouraged the purchase of domestic. Second, to identify and shame people of lower social status or various stigmatized groups. Requiring prostitutes to wear striped hoods to mark themselves out, as a dirty/shameful profession, was in part an attempt to discourage other women from trying prostitution as a way to make money, and to remind everyone that prostitutes were "less than."
When prostitutes did try to dress to attract customers, the interesting thing is, a lot of times they did so by partially aping or completely adopting male clothing, or at least the clothing styles favored by young men. This was because men's clothing in the 15-16C was much more form-fitting than women's.
I also happen to think the logic of "red to hide blood" (how young does your friend think prostitutes were/do they think prostitutes could lose their virginity over and over for a set number of years?) or "green to hide grass stains" (so many lush lawns in medieval cities) is silly, but that's not really a historical point in and of itself. What is, as discussed above, is that sumptuary laws did not operate on that sort of basis or with that sort of purpose.