What is the relationship between gl_Color and gl_FrontColor in both vertex and fragment shaders

I have pass-through vertex and fragment shaders.

vertex shader

void main(void)
{
    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}

fragment shader

void main(void)
{
    gl_FragColor = gl_Color;
}

Those produce empty rendering (black not background color like glClearBuffer does).

If I modify the vertex shader to set the gl_FrontColor to gl_Color it does render untouched OpenGl buffer ... with is the expected behavior of pass-through shaders.

void main(void)
{
    gl_FrontColor = gl_Color; //Added line
    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}

I am confused, how settings the gl_FrontColor in the vertex shader can change the value of the gl_Color in the fragment one ? What I am missing ?