If you want to temporarily dye your hair a crazy colour like green, you can save a lot of money by making the dye yourself. Any colour that is available in powdered drink mixes like Kool Aid, including green, can be mixed with a few simple household ingredients to produce a quailty temporary dye. While temporary green hair dyes don't take a long time to make, it's best to set aside as much time as you can when using them because the dye takes a long time to penetrate the hair to make it vibrant, especially with dark hair.
Put on a pair of latex gloves. This will prevent the homemade dye from staining your hands.
Add two packets of unsweetened, green Kool Aid into a bowl. Slowly add some hair conditioner to the bowl, mixing it with a spoon as you go. Only add a little bit at a time until the mixture is thick like a paste.
Apply the mixture to your hair, using a comb to evenly distribute it from root to tip. Massage the mixture into your hair. Do not rub it into your scalp.
Pile your hair on top of your head and cover with plastic wrap. The wrap will cause a heat reaction that will allow the colour to penetrate deeper. Allow the dye to sit for as long as you can, as long as overnight if possible. The longer the dye is able to sit, the brighter the colour will be.
Shampoo your hair like normal and rinse until there are no traces of the dye mixture left and the water runs clear.