How to get iMovie to recognize blue in footage instead of green when using green screen effect?

Under the Green/Blue Screen effect, there's no option to choose whether it recognizes green or blue, and in a shot where I want it to use an area of blue, it keeps recognizing the grass in the background instead.

This is particularly annoying because I tested it with a different shot and it used the blue, so I'm really not sure what determines which one it scans for. If I put something in for the grass, export it, and use the effect again, it recognizes the blue perfectly (but now the grass is another random color). This seems like a problem with the software.

Is there any way for me to get iMovie to switch from green to blue? I can't find anything about this anywhere online.