In the 1800s, chemists discovered para-phenylenediamine (PPD) and its use in the creation of synthetic dye. At the same time, it was found that hydrogen peroxide was a gentler and safer chemical for hair bleaching. These two discoveries paved the way for Eugene Schueller, who created the first commercial chemical hair dye, which he christened "Aureole."
That product would go on to be known as "L'Oreal." The double-process for dying hair blond soon followed, and in 1932 hair dye was further refined by Lawrence Gelb who created hair dye that actually penetrated the shaft of the hair. His company would be called "Clairol." Later, in 1950, he would introduce the first one-step hair dye product that actually lightened hair without bleach. This ushered in the modern era of hair dye, including the ability for hair to be dyed at home.
From "The History of Hair Dye Colors" by Noreen Braman
A lot of people consider the brands L'Oreal and Clairol to be non professional. The fact as stated above is that they were the first to discover the types of hair coloring that we use today. Before hair color used to be a coating of metallic salts or vegetable hennas that had little range and did little to cover grey.
With the discoveries by L'oreal and Clairol we have the beginnings of what we use today. Today's hair colors have been refined even more to make changing color and cover grey easier and with better results as well as offer us an endless range of shades that could be mixed.