https://www.gov.uk/government/news/first-allergy-test-for-hair-dye-to-go-on-general-sale
Colourstart test patches are listed on the U.K. govt’s medicine pages as a non prescription medicine and now available for the general public to buy directly. They contain the common ingredients found in hair dye and state that their own insurance covers you if the client reacts to a professional colour after using one of their patches and it testing negative.
I haven’t used them since I moved to Ireland as they’re not sold over here, but I used to use them regularly when I lived in the U.K. and worked mobile. They’re very simple for clients to use.
Nowadays, I test with the brand of tint I’m using, but in a very dark shade, usually a 3 as that will contain more PPD or whatever chemical is used for creating the colour in the tint.