No matter what kind of paint you have, you will use the same general technique to darken green paint. When mixing paints, add very little at a time, as it's difficult and sometimes impossible to salvage a colour that has been tinted too much. While black may seem like the most effective colour to use to darken green paint, it may actually make the colour murky.
Add a very small amount of blue paint to the green paint.
Mix the colours together using your paintbrush and determine if your green paint has been darkened enough. Only add more blue if the paint has been blended completely.
Brighten the shade and take away the blue tinge by adding a touch of yellow paint, stirring it in with the same brush.
Make sure to mix all the paint you are going to need for your project the first time around, as it's difficult to recreate the same shade.