Before the glutMainLoop I define the lights and material colors:

Code :
    glShadeModel(GL_FLAT);
 
    glMaterialfv(GL_FRONT, GL_AMBIENT, (const GLfloat[]){1,0.2,0} );
    glMaterialfv(GL_FRONT, GL_DIFFUSE, (const GLfloat[]){1,0.6,0} );
    glMaterialfv(GL_FRONT, GL_SPECULAR, (const GLfloat[]){1,0.8,0} );
    glMaterialfv(GL_FRONT, GL_SHININESS, (const GLfloat[]){1,1,0} );
 
    glLightfv(GL_LIGHT0, GL_AMBIENT, (const GLfloat[]){1,1,1});
    glLightfv(GL_LIGHT0, GL_DIFFUSE, (const GLfloat[]){1,1,1});
    glLightfv(GL_LIGHT0, GL_SPECULAR, (const GLfloat[]){1,1,1});
 
    glEnable(GL_LIGHT0);
    glEnable(GL_LIGHTING);

And that's ok, but now I need to change the color material at runtime, when the user presses a key.
I still haven't studied shaders so I think that I'll first do it this way, and study shaders later because I need to finish a project in short time.
So when the user presses a key I just change the material this way:

Code :
    glMaterialfv(GL_FRONT, GL_AMBIENT, (const GLfloat[]){0,0.2,1} );
    glMaterialfv(GL_FRONT, GL_DIFFUSE, (const GLfloat[]){0,0.6,1} );
    glMaterialfv(GL_FRONT, GL_SPECULAR, (const GLfloat[]){0,0.8,1} );
    glMaterialfv(GL_FRONT, GL_SHININESS, (const GLfloat[]){0,1,1} );

I may also change only the ambient light or whatever, depending on the key pressed,After this I call glutPostRedisplay().
At the start of the program all is fine and I see a teapot drawn with glutSolidTeapot, being displayed with the right material.But whenever I press a key instead of changing the material, the teapot disappears, why?