That depends on whether you are mixing paint or light.
In light, colors are addative. Red light is red, green light is green, red mixed with green is red light plus green light. Our eyes work with 3 kinds of cones, each of which senses a different point on the light spectrum. Since our cones each senses either red, green, or blue, those are the primary colors of light.
So when referring to the light, red and green mix to form yellow, the secondary color halfway between them.
In pigment (such as paint) however, colors are subtractive making things more complicated. The red pigment absorbs everything but red light which it reflects and the green pigment absorbs everything but green light which it reflects, and if you mix the two you’ll get a color that absorbs some green and red light and reflects some green and red light. Because of this, mixing pigments is not exact and does vary based on what the pigments are made of and mixed with. That being said, ideally, the primary colors of paint are yellow, cyan, and magenta. However, as I said, paint composition matters for this, and many people follow Sir Issac Newton’s model of the color wheel with red, blue, and yellow being the primary colors. That being said, in general mixing red and green paint will result in a shade of brown or dark orange. If your medium fits closer to Newton’s color wheel, you will likely get a more vibrant brown whereas if your medium closer resembles an ideal medium, the brown you get will be darker.