i think the hanky is still somewhat used in bdsm. like, if you see dudes in leather at pride and they have different color hankies, chances are they're using the hanky code.
it's a good visual representation when you're out and about, but then it requires others to know wtf every color means and which pocket it is. also, there's different sources on what the colors might mean. so in one decoder white means vanilla, on another it means masturbation only(as in, bate buddies).
for an in person signifier, if you live in a less progressive area, painting your nails is a good choice. but if you live somewhere that's super lgbt friendly and the lines between what gay and straight people style themselves are paper thin, not so much. you'll find straight dudes with painted nails just because they like it. in the midwest tho, it's a good way to let everyone know you like the cock.
when it comes to online signifiers, most people just use emojis. like they'll have the up arrow to say they're a top, the eggplant or horse to say they're hung, water droplets if they're into piss, poo emoji for scat, diamond if they're into pnp, dog if they're into pupplay, devil if they're super kinky, etc.
i think for mark, who spent a long time trying to be as straight passing as possible, wearing nail polish is a big step. i think he realized he was somewhat alienating himself from connections in the community by trying to be as straight passing as possible, and now that he's trying to find a community in a place where it's lacking it, he finds himself trying to use aspects that are deemed more feminine, such as painting your nails, so he can be seen as queer and be able to be identified by other members of the community.
solely based on what we see, he seems to spend most of his time by himself, with his dog, visiting his parents, or with donny. he might be lacking in friends and might be having a harder time to make queer friends than when he was living in l.a. so he might be trying to push himself to be less straight passing in hopes to find some friends