Allergic Reaction / Contact Dermatitis complaint would often come from an individual who has recently either had a colour carried out at the hairdressers or perhaps used a home colorant. This reaction may take effect as soon as the product touches the scalp, a few hours after or even the day after the colour. This can make the scalp feel hot, itchy, look red and become very sore. These feelings are produced by a degree of inflammation on your scalp caused by the product(s). In extreme cases this can even cause an anaphylactic reaction, however, this is rare. Most often contact dermatitis produces a mild reaction which presents like an eczema type of reaction to the scalp.
It is essential that before you have a colour at a new salon or try a brand of colour at home that you carry out/request a patch test. This is when a small amount of the product is applied behind your ear and left for 24 hours with a plaster over to monitor for a reaction. In addition, a small strand of your hair should be tested to see how it reacts to your chosen colour.