I used Soft Pastels by Artists Loft. You can buy them at Michael's. Use a spray bottle and saturate the strand of hair with water. Then grab a soft pastel and run it up and down the strand of hair. The first few times with each pastel takes awhile. Eventually they will wear and develop grooves - then it goes faster. With my dark hair, the bright pink, bright green and bright blue show up the best.
SO doing this next summer!