If you don't want to mess with Kool Aid, try this brand called Manic Panic. I've used it to dye my hair pink. If you bleach your hair first, obviously the color will be better, but if your hair is naturally light it should be good too!
I don't think they have it at Walmart, but if you call any of your local beauty supply stores, they can point you in the right direction.
A natural alternative, henna, but it doesn't come in funky colors.