glBegin(GL_QUADS)
    glColor4f(1.0, 0.0, 1.0, opacity) # back purple
    glVertex3f(x - r, y - r, z - r)
    glVertex3f(x + r, y - r, z - r)
    glVertex3f(x + r, y + r, z - r)
    glVertex3f(x - r, y + r, z - r)
    glColor4f(1.0, 1.0, 0.0, opacity) # front yellow  
    glVertex3f(x - r, y - r, z + r)
    glVertex3f(x + r, y - r, z + r)
    glVertex3f(x + r, y + r, z + r)
    glVertex3f(x - r, y + r, z + r)
    glEnd()