If you mix red and green, you will get a variant of brown. The exact shade of brown will depend on the quantities of red and green you mixed. For example, if you mix an equal quantity of red and green, you will get a shade of yellow-brown. On the other hand, if you add more red than green, you will get a darker shade of brown.
This is because when light is reflected off of a material, it is absorbed in different amounts depending on the surface color. When red and green are combined, they absorb most of the wavelengths of light that make up the colors red and green and reflect the longer-wavelength colors of yellow and brown.
Doing this in the digital world, such as on a computer, is much like paint mixing. If you have a digital photo-editing program, you can mix and create different shades of brown by adjusting the values of the sliders next to the red and the green color swatches.
Additionally, here is a helpful link from Skillshare that covers the topic in more detail:
https://www.skillshare.com/classes/What-Happens-When-You-Mix-Red-and-Green/1200255963?via=browse-discovery&enrollRedirect=1