We talk about hair colouring instead of hair dye.
What is the difference between chemical hair dye and ColourWell hair colouring? 

ColourWell Hair Colouring

Differently from chemical hair dying the result with natural hair colouring depends on your own hair colour. Natural colouring agents do not damage the hair structure. They apply a protective layer around the hair and do not damage the hair shaft in this way. Thin hair gains volume and strength, as every single hair gets more volume. The hair feels supple and glossy. Regular colouring with ColourWell gives your hair a longer lasting and intense colour

Chemical Hair Dyes

Most permanent hair dyes use a two step process. The first step is to remove the natural hair colour and the second step is to apply the new hair colour. With peroxide, ammonia and colouring agents the hair shaft opens, your own natural hair colour will be removed and the new colouring pigments (like PPD) are able to attach itself to the cortex. If this process will be repeated several times the hair loses its strength and the hair will be permanently damaged. Besides these hair dyes often contain carcinogenic paraphenylene-diamine (PPD). Increasingly people are developing allergic reactions to this dying agent, which often causes severe skin irritation.