Most hair color products today have nicer smells than the tell-tale rotten-egg odor that once accompanied permanents or hair coloring. And most color can be applied easily: some to wet hair, others to dry hair, worked into a shampoo-like lather, left to process and then rinsed（冲洗） and conditioned.
The down side is still that chemicals in hair coloring can be harsh and harmful to your hair if you don't know what you're doing or if you color or perm too often. How peroxide（过氧化物） and ammonia（氨） react with your hair is directly related to the level and kind of product you're using. Here are basic descriptions of the three major hair coloring product levels:
• Level 1, semi-permanent color -- This product adds color without changing natural color dramatically. The hair color contains tiny color molecules that enter the hair's cuticle, or outer layer, and go into your hair's cortex（头皮）. They don't interact with your natural pigments. And since the molecules are small, they eventually exit the hair shaft（发干） after several shampoos, leaving the hair as it was before treatment. This level generally lasts for 6 to 12 shampoos, covers up to 50 percent gray, enhances your natural color and leaves no roots. This hair coloring won't lighten your hair color because it contains no ammonia or peroxide.
• Level 2, demi-permanent color -- This product level lasts longer, through 24 to 26 shampoos. In this process, pre-color molecules penetrate the cuticle（表层） and enter the cortex（皮层） where they then partner to create medium-sized color molecules. Their larger size means they take longer to wash out. These products do not contain ammonia so the natural pigment can't be lightened. However, it contains a small amount of peroxide, which allows for a subtle, but noticeable, color enhancement. It also blends and covers gray. (Both semi- and demi-permanent colors can become permanent on permed or already-colored hair!)
• Level 3, permanent color -- This is what you need for a more significant color change (to go from black to blond, you'll still need to go with a process called double process blonding and it'd be wise to get this it done professionally). In this level, both ammonia and peroxide are used. Tiny molecules enter all the way into the cortex, where they react and expand to a size that cannot be washed out. Your hair actually has to grow out over time. This product acts to lighten the hair's natural pigment to form a new base and then to add a new permanent color. The end result is a combination of your natural hair pigment and the new shade you chose. That means the color may appear different on you than on someone else using the same color. Regular touch-ups of 4 to 6 weeks are generally needed to eliminate roots -- hair with your natural color growing at half an inch per month from your scalp（头皮）.
Now that we've reviewed the different product levels used in hair coloring, let's look at what actually happens to your hair. For example, if you're blonde and are going darker -- to brown -- permanent hair color uses the interaction between the ammonia and the peroxide to create a new color base in your hair shafts. If you go in the opposite direction -- from black or brown to blonde -- the hair goes through an additional step. First, bleach is used to strip the color from the hair. Then the ammonia-peroxide reaction creates the new color and deposits it in the hair shaft. If you use a semi-permanent color, the hair is coated with color, rather than deposited into the hair shaft.
(来源：howstuffworks.com 实习生吴昭文 英语点津 Annabel 编辑)