Why does the lighting change the objects color?

Question

I have a code that draws a sphere. Without lighting it is white, but if I enable lighting, it's drawn in gray.

I don't know why the sphere changed it's color

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>

void init(void) 
{
    GLfloat mat_specular[] = { 1.0, 1.0, 1.0, 1.0 };
    GLfloat mat_shininess[] = { 50.0 };
    GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };
    glClearColor (0.0, 0.0, 0.0, 0.0);
    glShadeModel (GL_SMOOTH);

    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
    glLightfv(GL_LIGHT0, GL_POSITION, light_position);

    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glEnable(GL_DEPTH_TEST);
}

void display(void)
{
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glutSolidSphere (1.0, 20, 16);
    glFlush ();
}

void reshape (int w, int h)
{
    glViewport (0, 0, (GLsizei) w, (GLsizei) h);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity();
    if (w <= h)
        glOrtho (-1.5, 1.5, -1.5*(GLfloat)h/(GLfloat)w,
                 1.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);
    else
        glOrtho (-1.5*(GLfloat)w/(GLfloat)h,
                 1.5*(GLfloat)w/(GLfloat)h, -1.5, 1.5, -10.0, 10.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize (500, 500); 
    glutInitWindowPosition (100, 100);
    glutCreateWindow (argv[0]);
    init ();
    glutDisplayFunc(display); 
    glutReshapeFunc(reshape);
    glutMainLoop();
    return 0;
}

Answer 1

You haven't set a diffuse colour for your light (via glMatrialfv (GL_DIFFUSE, ...) and - according to the GL spec - the default diffuse colour is {0.8, 0.8, 0.8, 1.0} - i.e. light grey. You also haven't set an ambient colour (default {0.2, 0.2, 0.2, 1.0} - dark grey).

The default behaviour of lighting is that it multiplies by the current colour, so with an object that's coloured white, that's going to result in grey.

In other words, it's grey because you told it to be grey.