By default, PhotoKey Pro uses a Chroma Key which auto-selects a color for removal. This tool examines the entire image, to find the single color that will remove the greatest portion of the image. In most images, this works quite nicely to determine the appropriate color for removing the green screen. But in rare cases, it might select the wrong color.
If you find that PhotoKey has auto-selected the wrong color, you can correct the issue by manually selecting the color you desire to remove. Do this by locating the eyeropper tool in the Auto Chroma Key controls, and dragging the eyedropper onto your image, to select a specific color. A second option is to switch from Auto Chroma Key to Auto Color Difference Key, to see if it handles your specific image better.