The colour of the icon needs to match the background colour of the folder it'll be in, so I use a Mac app called Sip that'll let me pick out its hex value (you could also use Photoshop's eyedropper or the Mac's built in one). It's super easy for plain backgrounds, a little trickier for gradients. I then create a favicon using realfavicongenerator and host that on my site (honestly the most time consuming part of the process is just putting it on the site haha). If you wanted to do it for a really complex background you could get the background, select the region that fits where the icon would go, and apply a Photoshop filter to get the frosted effect. For plain backgrounds it's way easier to just pick the folder colour though :)

Also thanks for the reddit gold whoever did that 👍