The one thing I tend to do when I am trying to extend the palate to use with given colors like red and green is to use a color wheel and see what comes up in common for both of them. The grey is a good choice and so is white. Both pull out the other two colors and make them pop. A color wheel is very inexpensive if you don't have one and can open a new world of color choices that you may n ever have considered. Best of luck.