I think he likes the various shades of purple, violet, and magenta.
But the main reason is that magenta is the opposite color to green so the two look good together. (that’s why the background of his picture is magenta)
Ah, so it’s from Arakawa Under the Bridge? That’s interesting; I’ve seen that anime years ago, but don’t remember that joke at all (it’s funny how some moments from the things we watch or play and don’t think too much of can later turn into huge memes). But yeah, I doubt Flourish would be into a samurai-style haircut (although he does dress like a medieval prince, so there is some similarity )
I think most of them are self-explanatory (and also I didn’t really put that much thought into them, honestly), but if you have a particular one in mind, ask away!
Also, the outfits are subject to change if people have some ideas about what they should be.