Add a small amount of black paint to green paint
Add a small amount of blue paint to green paint
Add a small amount of brown paint to green paint
Add a small amount of dark green paint to green paint
Mix in a tiny amount of red paint to mute and darken the green
Use less white paint or avoid adding white
Increase the ratio of green pigment to lighter colors
Mix in a little complementary color to reduce brightness
Test the mixture in small amounts before adding more
Adjust gradually until the desired shade is reached