There isn’t one universal “best” color. For chroma keying outdoors, choose a bright color that is strongly different from both the background and the subject in front of it.

In a forest, green is usually a bad choice because it matches the surroundings. A bright primary color is often easiest to key because one color channel will separate well; red or blue may work better than green in this case. Bright orange can also work if it gives stronger contrast against the scene.

A good rule is to pick the color opposite to the dominant scene color. In a green forest, that usually means leaning toward red/orange rather than green.

If the edges are semi-transparent, soft, or out of focus, a traditional chroma key can create halos. In those cases, using a color closer to the intended final result—or even using a high-contrast manual mask instead—may work better. White can be effective if it stands out well from the forest and you plan to create a mask rather than rely purely on chroma keying.