Koolaid

On brown hair, you'll be lucky if Kool-aid shows up at all, and it'll wash out after just one or two washes. The only way to make Kool-aid show up well and stay in a long time is to bleach your hair first. The lighter you get it, the brighter the Kool-aid will show up.
I've used Kool-aid twice before. Once I used green on my natural hair for St. Pat's, and it only showed up well under fluorescent store lights. It washed out the very next day with no problem. The other time I bleached my hair (though my natural hair is brown, so the bleaching only got it strawberry blonde :-( ) and put blue on it. . . which unfortunately also looked green :-( . It showed up well under all kinds of light (even though it didn't look the right shade) and it stayed in there like forever.