Why do fragment shader colors an image based on the fragment coordinate as a saturated color?

The following code colors a image based on its fragment positions:

Vertex shader:

varying vec2 vXY; 	
void main(void) {
	vXY = position.xy;
	gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
}

Fragment shader:

precision mediump float;
varying vec2 vXY; 		
void main(void) {	
	vec4 color = vec4(0.0, 0.0, 0.0, 1.0);
	color.x = vXY.x;
	color.y = vXY.y;			
	gl_FragColor = color;
}

It separates the image into 4 squares of colors, as the picture bellow:

From this, I can clearly see the cartesian coordinate system, which is fine, but what I don't understand is: why is the brightness of the colors as if it were constant 1.0?

From my understanding, since the range of the (x, y) coordinates is from -1 to 1 in the real numbers domain, then each square of color should get gradually brighter as the position changes, but this does not happen. Why?

The "position" attribute contains the original coordinates of the vertex specification. The components of that attribute are only in the range [-1, 1] if you have specified them in the range [-1, 1]. The normalized device coordinates are in the range [-1, 1]. You get the normalized device coordinate by dividing the xyz components of the clip space coordinate (gl_Position) by its w component (Perspective divide):

varying vec2 vXY;   
void main(void) {
    gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
    vXY = gl_Position.xy / gl_Position.w;
}