We provide a hair color chart as below for your convenience. If you know what color you want and you're going to have them colored and styled after you buy them, then this may work for you. You can see all the colors online right there and make a choice and be on your way. No waiting for samples and color charts to get the job done.
If you are going for a wild color (i.e. shocking pink) or want to add extra highlights, then you could just buy that online without matching it up. For example, if your hair has a blend of colors in it, it's better for you to choose the middle tone color by blending with the medium tones so everything will flow together. For adding in highlights, simply add in a few lighter extension pieces.