I am experimenting with the JavaFX Canvas and noticed strange output when filling polygons. It seems to be related to whether or not a clip is applied. This minimal(-ish) test case reproduces the problem for me on MacOS JDK1.8.0_05. With the CLIP flag set to false, the output is what I expect. Changing CLIP to true triggers the bug (the polygon fill shows two shades of gray for me, instead of one):

import static javafx.application.Application.launch;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

// A test case showing a polygon fill on JavaFX canvas. The fill should have
// a single color, but when clipping is enabled the fill is corrupted and
// shows more than one shade of gray.
public class TestCase extends Application {
    
    // change this flag to TRUE and the shape fill is corrupted
    // at least for me on MacOS JDK 1.8.0_05
    private static final boolean CLIP = false;
    
    // based on ResizableCanvas:
    // http://dlemmermann.wordpress.com/2014/04/10/javafx-tip-1-resizable-canvas/
    static class TestCanvas extends Canvas {

        public TestCanvas() {
            widthProperty().addListener(evt -> draw());
            heightProperty().addListener(evt -> draw());
        }
    
        private void draw() {
            double width = getWidth();
            double height = getHeight();
 
            GraphicsContext gc = getGraphicsContext2D();
            gc.save();
            gc.clearRect(0, 0, width, height);
 
            if (CLIP) {
                gc.beginPath();
                gc.rect(0, 0, 600, 400);
                gc.closePath();
                gc.clip();
            }
        
            gc.translate(300.0, 100.0);
            gc.beginPath();
            gc.moveTo(-112.82283020019531,203.73513793945312);
            gc.lineTo(-254.63790893554688,30.293907165527344);
            gc.lineTo(45.6809196472168,27.10471534729004);
            gc.lineTo(230.38345336914062,177.9417724609375);
            gc.closePath();
            gc.setFill(Color.rgb(184,184,184,0.4));
            gc.fill();

            gc.setStroke(Color.RED);
            gc.stroke();
            gc.restore();
        }
 
        @Override
        public boolean isResizable() {
            return true;
        }
 
        @Override
        public double prefWidth(double height) {
            return getWidth();
        }
 
        @Override
        public double prefHeight(double width) {
            return getHeight();
        }
    }
 
    @Override
    public void start(Stage stage) throws Exception {
        TestCanvas canvas = new TestCanvas();
        StackPane stackPane = new StackPane();
        stackPane.setPrefSize(600, 400);
        stackPane.getChildren().add(canvas);
 
        // Bind canvas size to stack pane size.
        canvas.widthProperty().bind(
                       stackPane.widthProperty());
        canvas.heightProperty().bind(
                       stackPane.heightProperty());
 
        stage.setScene(new Scene(stackPane));
        stage.setTitle("Test Case");
        stage.show();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
    
}