Anea already has explained some of the sumptutary laws regarding prostitutes or courtesans being ordered to wear yellow scarves but here are a couple more sources.
However, since many of the portaits I found of ladies with some sort of yellow scarf are before this date, I also found this footnote:
So there is some connection of yellow veils/scarves and courtesans - one that wasn't always followed based on the numerous complaints of the day. However, what about green? Green, it turns out, doesn't have a negative association in the 16th Century. Rather, it stands for chastity according to Lisa Jardine's book Worldly Goods (page 15). This could be seen two ways - the portraits I'm seeing particularly in the 1520's and 1530's of what I believe to be courtesans are all just playing a practical joke (very, very period. It was so common that several individuals lamented at the time that courtesans were dressed up as noblewomen or widows -even nuns- causing the actual noblewomen of Venice and Rome great distress)or it's just a coincidence that these ladies are wearing the same shade of green with a yellow scarf. I think it's the former rather than the latter.
Green also stood for one of the three cardinal virtues. But it could also mean love which makes a lot of sense in the context of it being worn by a courtesan. The idea of it being a symbol of love is re-enforced much later during the Baroque era.
With this in mind, here are some of the portraits I've been looking at.
Mary Magdalen is easy to spot in any period painting as she is normally shown with a perfume jar, as in the painting above. She is also commonly accepted to have been a prostitute prior to admonishing her ways and following Christ - therefore, she is often painting wearing flashy to somewhat scandalous clothing for the time. Knowing that, I realized that the yellow scarf might be the yellow scarf of the courtesan. However, I didn't think about the green dress until I kept seeing this kelley green and a yellow scarf together in multiple paintings.
Again, Mary Magdalen is pretty easy to spot. She's wearing a dress more closely associated with southern Italy in this painting, however.
The lady, not having a bodice to her gown in the same kelley green as both the Mary Magdalen's, is most likely a courtesan. She is shown, surrounded by men, playing a game - very much what a courtesan would do. Her scarf is a harlequin of yellow and white - probably acceptable enough to pass for any law.
Again, the yellow scarf with the same shade of green.
This green is a bit lighter - however that may be due to age. Still, she has the yellow scarf about her shoulders.
It looks like she has a light yellow scarf about her shoulders. Again, the bright green dress.
Of course, it is possible that this was simply a fashion statement - however, there are paintings of Mary Magdalen wearing a yellow scarf with a green dress, known laws at the time instructing courtesans to wear yellow scarves, and evidence that green meant love.
I'll keep a look out for more paintings from this era to see if I can find anything more solid - but I do believe that at least around the 1530's in most of Italy, there was something to green being worn by courtesans.