What color do you get if you mix red yellow and green?

If you mix paint of those colours, you will get a brown-black mixture. This is because paints work through absorbing certain colours of light, and mixing all three will result in a mixture which absorbs all of the colours and reflects none or very little.
If you mix light of those colours, red and green form yellow, so you would get a yellow colour. Yellow because red + green = yellow and yellow + yellow = yellow.