Take a high-resolution photograph of your existing cabinets. Print out the photograph on high-quality paper and take it with you when selecting your thermofoil doors.
Hold the image up to the thermofoil samples to match the colors.
Take samples of thermofoil home, selecting a few shades of thermofoil cabinet doors in a color that matches your existing cabinets. Hold the samples up to your existing cabinet fronts to match the colors.
Eliminate colors that are too dark or too light for your cabinets. If you can't locate a color of thermofoil that matches your cabinets exactly, choose the color that most closely coordinates with your current cabinets.