How do you do that?
The hair color instructions tell you to always put some of the mixture on the inside of your elbow or some other “discreet” place and wait 24 hours. If no rash develops it’s supposed to safe to use.
I’ve colored my hair many, many times and, I admit, I never do the test. But I also had a terrible allergic reaction to eyebrow waxing last year (something I’d done many times as well, with no prior problems) so it can definitely happen “all of sudden”.
I should add that “beauty products” are notorious for changing their ingredients with no warning, so it’s really no surprise that allergic reactions happen even if someone has used a product multiple times in the past with no problem.
I don’t use hair dye but friends do - they’d put a bit on their wrist or arm to see if there’s a reaction. They always stick to a product they know and it’s only for the occasional hair lightening not a color change.