The value of a colour refers to how light or dark the colour is, and it is easy to change the value of any colour you like. If you find yourself with a shade of green that isn't as dark as you want, there is no need to return to the paint store; adjust the colour yourself.
Place the amount of green paint needed for your project on a paint palette.
Add a small amount of black paint to the green and mix thoroughly with a paint brush or mixing stick. Your green paint will become darker.
Add more black paint and mix thoroughly again until you reach the desired shade.
Add black paint a little bit at a time so that you don't create a shade darker than you wanted. If your paint becomes too dark, add more green to lighten it. A colour wheel is a useful tool to have when mixing paint.