I just wanted to put it out there that you MUST do this.
This week my OH decided he wanted to experiment covering his greys, and bought a hair dye. He was going just put it on, but decided to do a patch test on his arm, and within a few hours it had swelled up. Its was a big red, sore, angry and swollen welt, and days later it still is.
Obviously he didnt use the dye, and I doubt he will ever use any ever again. But its not worth thinking about what might have happened if he'd not done the test.
I have started dying my hair, and have swapped brands to find the right colour for my hair - and been so tempted to not test each time - but I do and will now every time.
So please make sure you're doing this test ladies!!!