Hello,

With very few details one can only imagine the scene when you were at the hair salon.You did mention your allergy when itching started? You recall having being told your hair be exposed to the chemical color for 20 minutes which indeed was extended by another 15 minutes...

Two ways to deal :

amicable agreement or with judicial assistance

Make sure you have serious notes on the event chronology and keep the practitioner's report as proof of your post-treatment allergy .

Do you consider that the hair salon is responsible for your allergy?  Or are you saying that they should have known you had an allergy?  Or are you saying they used a bad (non tested) product.  Before taking action you need to be clear as to why you think they reacted wrongly, or we're negligent.