Floor color different than background color - lighting

Hello there!
I have a scene. In this scene, I turned off all the lights to see if it was causing this issue… It appears to be.
Here is the problem I am having. I am setting scene background color like so:

    scene = new THREE.Scene();
    scene.background = new THREE.Color( 0X999999 );

Then I set up some geometry to act as my “ground” or “floor”, like so:

    const mesh = new THREE.Mesh( new THREE.PlaneGeometry( 60, 60 ));
    mesh.rotation.x = - Math.PI / 2;
    mesh.receiveShadow = true;
    mesh.material = new THREE.MeshStandardMaterial( { color: 0x999999 } )
    scene.add( mesh );

So as you’d expect, the ground is black. Lets add some ambient light. Around .55 intensity the ground is pretty close to the background color to create that seamless impression

    const ambientLight = new THREE.AmbientLight( 0xcccccc, 0.55 ); // soft white light
    scene.add( ambientLight );

Right, so far so good. But wait, we want to cast some shadows from our models on to this ground of ours

    // Shadow casting light
    const dirLight = new THREE.DirectionalLight( 0x999999, .55 );
    dirLight.position.set( 3, 25, 10 );
    dirLight.castShadow = true;
    dirLight.shadow.camera.top = 30;
    dirLight.shadow.camera.bottom = - 30;
    dirLight.shadow.camera.left = - 60;
    dirLight.shadow.camera.right = 60;
    dirLight.shadow.camera.near = 0.1;
    dirLight.shadow.camera.far = 2540;
    dirLight.shadow.mapSize.width = 1540
    dirLight.shadow.mapSize.height = 1540

    scene.add( dirLight );

And here is where it all goes wrong! The reflection of the directional light off the ground is causing the whole ground mesh to brighten. It looks like a big square in the middle of nowhere.

So what am I actually trying to do here?
I would like a “ground” that you can cast shadows upon
I would like a “background” that exactly matches this ground.
The impression should be of a single color seamless room or box.
Anybody have any suggestions? I tried to make the dirlight color to the same color as the background too.

I would consider using fog instead of just a solid background color. Then your floor can fade out in the distance smoothly, and you can light the scene without worrying about mismatched colors. Many of the official examples do something like this: