Green and orange combine to form brown.
According to Color Matters, both green and orange are secondary colours, which are created by combining two primary colours. Mixing any two secondary colours results in a brown hue, ranging from muddy brown to olive brown.
The exact shade of brown produced by mixing green and orange depends on the hues of green and orange. While light colours produce a light, yellowish brown, dark colours produce a darker, more olive brown. Mixing green or orange with a primary colour may produce a tertiary colour, but mixing the two secondary colours always produces brown, regardless of hue or brightness.