I wanted this look:
So I ended up getting a petticoat tutu from the Leg Avenue brand, this is especially useful because you can decorate it up any way you want. I got some colored sheets of sticky glitter foam from Michael's, then cut them into circles and sewed them on with a couple of stitches since the sticky backs weren't enough to hold them on.
And voila, there's the main part of your costume! Now you just need to add a blue wig, sparkly shoes and fun candy accessories and you're done!
Here was the final product the way I wore it:
And here are the sources for my main items!
*White Petticoat tutu: Party City
*Colored glitter foam sheets: Michael's or most any craft store
*Headband: Katy Perry Web store (not necessary but held my wig on nicely!)
*Candy Buttons Jewelry: I made my own and wrote a post about it last year, seen HERE.
*Glitter Heels: I wore silver sequin ones from Rampage, style name: Greer
*Blue Wig: I got mine from a random store here quite cheap but Party City has them and I've also seen them on eBay.
A note about the tutu petticoat, the tank top part is rather thin so I added another white tank top under it. I also suggest a pair of white bike/running shorts under the tutu as it's quite short.
I hope this helps you come up with a totally Katy costume for Halloween or other fun events!