Hair color is the rage. Why? Baby-Boomers. There are more people in the US with gray hair than ever before in history.
There are several ways to cover the gray:
With Permanent Color: Most permanent color uses peroxide (10 to 40 vol.) and ammonia. It covers gray the best. The drawback is re-growth of new hair. It is a different shade and will leave a line of "demarcation."
With Demi-Permanent Color: Uses peroxide in low vols. (5-7). Many have no ammonia or are acid type colors and will not damage the hair cuticle. This type of color will cover 60%+ of gray for 8+ weeks. The positive is that when the color fades, it does so gradually. It leaves no line of demarcation.
With Semi-Permanent Color: These are "fun" colors, meaning they will last a short time (1-4 weeks). They usually cause no damage to the hair. The best to expect is 40% gray coverage
With a Color Rinse (semi-permanent): These are fun but a waste. Go with a demi (I like to cut to the chase). Rinses are great as toners. They are great to cut down unwanted (brassy or ash)tones in the hair. For the money go for a demi or permanent color.
All About Hair Color
If you know the color wheel it becomes easy...
It may sound a bit odd but your hair is a mixture of 3 colors: red, yellow, and blue. These are the primary colors. Secondary colors are orange, green and violet. If you look at the "wheel", a color opposite (directly across) will "negate" that color. This means that if your hair is an orange color, green will make it a brown color. If your hair has a yellow tone, violet will cancel it out.
One of the most important elements of hair coloring is determining the hair's "underlying pigment". When you choose a color in a swatch book, your hair may not come out that color because of the underlying pigment in your hair.
Underlying color + Artificial color = Final result