Two years ago, my daughter got one of the worst cases of green slime hair that I've ever seen. I didn't want to waste a lot of money on the de-greening shampoo, but I thought I'd have no choice. I tried the tomato sauce and it did nothing. I googled for more ideas and came up with baking soda, so I tried it. It took 3 applications to get all the green out, but it worked!
Just wet the hair and lather it up with any shampoo. Then pour some baking soda on top and work it in. Keep adding baking soda until the hair is kind of thick with it. It should take about 1/2 of a small box. Let it sit for about 10 minutes, then wash it out. You'll definitely see immediate results, but you might have to do it 2-3 times before ALL of the green is out.