One option is to try out hair chalk like Free People Ombre Hair Chalk ($14 at freepeople.com). Each metal tin comes with five different shades of colors (choose from blue, black, pink, green and yellow) that apply easily to your hair for a fun effect. With hair chalk you decide how dark you want the color to be, and whether you want to just color the ends or the entire strand.
Or another, commitment-free way is to pick up colored extensions, like the Pop by HairUWear Clip-In Color Strips. They come in 6 fun colors, Amethyst, Blue, Gold, Pink, Purple and Red and are only $10 each at ulta.com. Fasten them in hair when you want them and leave them home when you don't!