The green color you see in your hair after swimming in a pool is likely caused by a reaction between the copper in the pool water and the protein in your hair, known as keratin. This reaction can cause the formation of copper complexes with the keratin, resulting in a greenish tint to your hair. However, it's important to note that the copper concentration in pool water is usually very low, and this green tint is usually temporary and can be easily washed out.

Here are some additional factors that can contribute to green hair from swimming:

1. Chlorine: Chlorine is a common disinfectant used in swimming pools to kill bacteria, but it can also react with the copper in the water and contribute to the formation of the greenish tint.

2. pH levels: The pH level of the pool water can also play a role. When the pH level is high (above 7.6), the copper becomes more soluble and more likely to react with the keratin in your hair.

3. Sunlight: Exposure to sunlight after swimming can intensify the green color in your hair, as sunlight can cause the copper complexes to oxidize and become more visible.

4. Blonde hair: Blonde hair tends to be more susceptible to this green discoloration because the pigment in blonde hair is lighter, allowing the green tint to be more noticeable.

5. Conditioning treatments: Some conditioning treatments can deposit a thin layer of mineral buildup on your hair, which can enhance the reaction between the copper and keratin, leading to a more pronounced green tint.

If you're concerned about the green tint in your hair after swimming, you can try the following tips to prevent or remove it:

- Rinse your hair thoroughly with fresh water immediately after swimming to remove as much chlorine and copper as possible.

- Use a clarifying shampoo to remove any mineral or product buildup on your hair.

- Apply a deep conditioning treatment to help repair any damage to your hair.

- You can also try using a purple shampoo or conditioner, which is designed to neutralize brassy or yellow tones in blonde hair and may help counteract the green tint.

It's worth noting that if you have persistent green discoloration in your hair, it may be a good idea to consult with a hair professional for advice and treatment options.