void processSpecialKeys(int c, int x, int y) {

    if (!flag) {

        if (c == GLUT_KEY_F1) {

            int x = glutGetMenu();

            if (x == menu1)

                glutSetMenu(menu2);

            else

                glutSetMenu(menu1);

            // don't forget to attach the menu!!!

            glutAttachMenu(GLUT_RIGHT_BUTTON);

        }

    }

}

void createGLUTMenus() {

    menu2 = glutCreateMenu(processMenuEvents);

    glutAddMenuEntry("Blue",BLUE);

    glutAddMenuEntry("Green",GREEN);

    glutAddMenuEntry("Red",RED);

    menu1 = glutCreateMenu(processMenuEvents);

    glutAddMenuEntry("Red",RED);

    glutAddMenuEntry("Green",GREEN);

    glutAddMenuEntry("Blue",BLUE);

    glutAttachMenu(GLUT_RIGHT_BUTTON);

}