When coming to the salon, understand the products your colorist uses
The hair color process is a chemical process that usually involves several chemicals that are considered to be toxic and even carcinogenic includingammonia, sulfates, parabens, formaldehyde, and resorcinol. The human scalp can be quite sensitive to these chemicals and they may even be quite harmful to the body.
Studies have shown that 60% of whatever you put on your skin (lotions, perfume, etc.) is soaked into the bloodstream through the skin's pores and follicles. Your scalp has more pores and follicles than any other part of your body. Therefore, you should be very careful and very aware of what chemicals you're applying to your scalp.
Prior to your appointment, find out the hair color products that your salon uses. Research these brands online and look into each of their ingredients. Search for ingredients that you may have a known allergy to. If you are unsure about any ingredient or cannot find any information on the ingredient then you should call the manufacturer. Manufacturers have Material Safety Data Sheets (MSDS sheets) that will provide a wealth of information on the ingredients. You should consult your physician if you have any concerns whatsoever.
While your professional colorist will follow all state laws required to guard against allergic reactions, you should be comfortable with the products you are using and do your best to avoid any specific allergies. If you have known allergies, a small amount of research will help avoid a very negative beauty experience.