Purple with blue inside is invisibility. You're half invisible to you, but really hard to see when you're walking to other people. When you are stopped, you're completely invisible. It's good if you see the person who's it before they see you.
Green with white on the inside is disguise. You turn into the reverse of what you are (if you're it, you are a regular player, and if your a player you look like you're it). It's good if you want to confuse the other person, and surprise players if you're it. It's also fun to get chased by an army of elves if you turn into a character in front of them.
Green with a yellow outside is teleport. It teleports you to a random place in your house. It's good if you need a fast getaway, and if you have no idea where to search for players if you're it.
Yellow with a red outside is speed boost. Speed boost makes you go a little faster. This is great if you want to run away fast, and if you are close to catching someone when you are it but just need a little boost.
Blue with a pink inside is freeze. Freeze freezes anyone near you. This is good if you have the it person tailing you (but not fun for friends who are also running away), and is a good way to catch a bunch of players at the same time if you're it.
White with a black inside is immunity/break immunity. Immunity lets you not be able to be tagged for a few seconds, and break immunity breaks immunity if you're it. I think this is the least helpful power up star, especially break immunity, but it can still be good in some situations.I hope this guide helps you decipher between the different power up stars. I also hope you enjoy tag as much as I do!