what color/colors should I use to achieve the color of a chalk board?

Kelly green would probably work

AI would start by adding a little white to your black it will either be a purple based black or green based. If its the latter your work is done. If its purple try adding a drop of green to it.

AUsually this is the color my buttercream comes out because theres not enough black food color. So I believe the Wilton cake color is the green based and my gel brand is the purple base.

I guess it depends on whether you want it black or green. The chalkboard in my classroom was green, as were most of the others in my school. You could search through the chalkboard cakes on here and ask them how they got the color of theirs.