In regards to the directional positioning of the color wheel the opposite hue of green will be as others have said, red. Keep in mind that red, yellow and blue are primary hues while green is considered a secondary one. Now within the secondary colors, technically green's opposite will be orange and purple
Complementary colors are any two colors which are directly opposite each other, such as red and green and red-purple and yellow-green.
Analogous colors are a group of three colors next to each other on a color wheel, such as violet, red-violet, and red.
Red. Red and green are complimentary colors, so they have a really high contrast. Also, depending on the green, you could get a nice contrast with black or white. White stands out nicely on a rich emerald or dark forest green.
