What is the resulting color if red and green are mix?

There are to possible answers to this question...
If what you are mixing is light (like the TV/Monitors does)... or if what you are mixing pigments (like inks, crayons, etc)...

In the case of light, Red Light and Green Light will give you Yellow Light... (for example when you are working with the RGB system you have to look at it like this... this is called Additive Colors)

In the case of inks, (for example the CMYK color model, that will be called Subtractive Colors)
The resulting color will be a shade of brown.