Red is the color that best complements green. On the color wheel, the colors directly across from each other are complementary to one another. Red is directly across from green.
Different shades of red complement different shades of green, not just the typical Christmas colors of emerald green and crimson red. An emerald green can be used with magenta red. Mint green can be used with a bold bright red. The idea behind using complementary colors is that the cones in the eye expect to see certain colors together. Staring at one color for a long time and then staring at a white wall allows the eye to show the complementary color in an afterimage.