What do you get when you mix red and green colours?

If you are using pigments, which are referred to as subtractive colors, like paint, crayons, etc, then you would get a shade of brown.

But, if you are using light, which are referred to as additive colors, like the ones used for TV screens, then you would get the color yellow.

When using acrylic paints, if you use a 3:1 ratio of red to green using cadmium red and phthalo green, you will get gray by adding varying amounts of white to it.