Look through the color swatches to see which colors grab your eye initially. Color swatches are samples of dyed synthetic hair. They're just like carpet samples, but for hair. You probably have some idea of what you want already, so you can limit your search to the blonds, reds or browns, or whichever color category you've got your heart set on. Narrow it down to a handful of colors that you like. Look at the color descriptions on the swatches. Weed out the cool colors if you've discovered that you're a warm-color person and vice versa.
Sweep your hair back off of your forehead and either hold your hair back with one hand or pin it in place. Fan the color swatches out over your forehead, close to your brow and eye. Choose the color that brings out the best tones in your skin and eyes. Also, look at the contrast between the color and your eyebrow. Do the colors go well together? They don't have to match perfectly, but they at least have to complement one another.
If none of the colors look right, broaden your search and try again. Go back to the swatches and select another set of colors to test against your skin. Open up your mind to colors that you may have overlooked initially. Maybe you need to rethink your whole color category or go a few shades lighter or darker.
Understand your skin's tonal qualities. You don't have to peer in the mirror for hours trying to figure out your skin's underlying tonal qualities, but you do have to have a basic understanding of what colors go best with your skin tone. Colors are broken down into three categories: warm, cool and neutral. Take a moment to think about what clothing colors look best on you. What makeup colors do you use? Are you a warm or cool person? If you wear warm colors well, you will look best with a warm hair color. If you're a cool-colored gal, you should lean toward cool hair colors. Neutrals work for everyone, because they're, well, neutral.
To choose a highlight color, separate a small strand of the color swatch and work it in with the rest of your hair so that you can see what a strand of the selected color will look like when blended with the rest of your hair.
What you see is not always what you get. When making swatches, hair color manufacturers apply the dye to colorless synthetic hair. What you're seeing is a true representation of what the dye does when no underlying color pigment is present. Your hair has tons of underlying pigment. It's not really a big deal if you're trying to go darker, but if you're trying to go lighter, your hair could take on a different tone than the swatch.