Get the separate color channels:
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
Then look where the blue and red channels are dark and the green channel is high.
mask = (redChannel < 50) & (blueChannel < 50) & (greenChannel > 50);
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask,class(rgbImage)));
You copyrighted the image to prevent download, so that's about all I can do.