Run this sample code :

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.MenuButton;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class MenuButtontest extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override public void start(Stage stage) throws Exception {
        Button button = new Button("Button");
        button.setStyle("-fx-text-fill:white;");

        MenuButton menuButton = new MenuButton("MenuButton");
        menuButton.setStyle("-fx-text-fill:white;");

        VBox vbox = new VBox(10, button, menuButton);
        Scene scene = new Scene(vbox, 200, 200);
        stage.setScene(scene);
        stage.show();
    }
}

You will see that the Label of the MenuButton is not white. If you use a simple "Button", the text will be white.