import java.awt.GradientPaint;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
import javax.imageio.ImageIO;

public class GrayIcon extends Application {
    @Override
    public void start(Stage stage) {
        Rectangle r = new Rectangle(100, 100, Color.GREEN);
        Group root = new Group(r);
        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.getIcons().add(new Image("icon.png"));
        stage.show();
    }

    static {
        File f = new File("icon.png");
        if (!f.canRead()) {
            BufferedImage bimg = new BufferedImage(32, 32, BufferedImage.TYPE_BYTE_GRAY);
            Graphics2D g2d = bimg.createGraphics();
            g2d.setPaint(new GradientPaint(0, 0, java.awt.Color.WHITE, 32, 32, java.awt.Color.BLACK));
            g2d.fillRect(0, 0, 32, 32);
            try {
                ImageIO.write(bimg, "png", f);
            } catch (IOException ex) {
                Logger.getLogger(GrayIcon.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }
}