If you want to go down the rabbit hole of trying to accomplish different tag colors using CSS, it might be possible, but I’ll warn you that it’s not easy. I’ve included a couple of threads below.
As for using emoji…it is the same as text, so you can copy, paste, or type it in any text box. I use IF columns to select different color emoji and text depending on different conditions. Then I just point the tag to that IF column.
You can also use different images chosen from an IF column as an image source for a tile or card list, or for the avatar.
There are a lot of ways to do it, but for simplicity, I think I would use emoji or images instead of CSS in this case.