WebGL get the current gl_FragColor

I am not very experienced when it comes to WebGL so any help will be greatly appreciated. I have drawn a simple square on screen and set the colour using gl_FragColor like this:

 fsSource = `
    void main() {
      gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
    }
  `;

And then passing the fsSource variable as a parameter to the initShaderProgram(gl, vsSource, fsSource);

The color is then changed later on. I would like to know if there exists some function which I can use to get the current gl_FragColor and be able to do something of the sort:

if ( getColourOfSquareFunction() == 'red' )
{
     //do something
}
else
{
    //do something else
}

Thank you!