Darkening green paint requires the help of a few different colors.
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 color that has been tinted too much. While black may seem like the most effective color to use to darken green paint, it may actually make the color murky.
Step 1
Add a very small amount of blue paint to the green paint.
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Step 2
Mix the colors 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.
Step 3
Brighten the shade and take away the blue tinge by adding a touch of yellow paint, stirring it in with the same brush.