It shouldn't be too much of a problem unless your hair color varies drastically in your horseshoe area from the sides to the back.

If your growing hair has very good density, you may be able to get enough hair from each different area for a sample. Generally, the more hair you send as a sample the better, but the factory puts the hair under a microscope to determine the color anyway, so I'd think you could probably get away with using slightly smaller samples for the different areas if you wanted to. Be sure to ask John if this will work before doing it though.

Another thing that could help is to choose two or three colors that match the closest from the color ring you have and then specify a mix of those colors for your order, but if I were you, I'd try to use my own hair if possible. (Always be sure to check the colors on the color ring against your own hair in full sunlight as well as indoor lighting to choose the closest color(s).

Hope this helps.