When sliding down a slide, you pick up a certain amount of static electricity. When rubbing the balloon against your hair (as you did in the experiment), you pick up a different amount of static electricity. It turns out that the amount of electricity in the experiment is significantly less than what you pick up when going down a plastic slide under the proper conditions. Since there is more electricity in that case, there is more energy. As a result, the color of the spark is different and it hurts you more.