It is important to choose the correct color for the chroma key. The color should match or closely match the background. The color displayed in the box is selected automatically by analyzing the current frame. Sometimes this is not the best color.
An effective method for chroma key color selection is to select the correct color by clicking in the original image where a cross-hair appears (to select a specific color/pixel), then use the Refine Key image to further refine the key as required. It's often best to click next to the edge of an object that is not going to be removed. Usually, the color near the object is the most effective chroma key color.
You can also click directly in the Refine Key window to adjust the selected color. The Refine Key color range displays the colors that are near (in value) to the currently selected color. (The Refine Key window does not display all colors in the image.)