At 5:06 you drop the cube, causing the bottom one to turn red. I copied your video EXACTLY, but my cube doesn't turn red. …

I commend you for using debug.Log, not a lot of people tend to do that and it almost always gives you an idea of what the issue is.

Now for OnCollisionEnter, that is a specific method so if it’s misspelled and/or not capitalized properly it will just treat it as a custom method. You can try it without the if statement as such:

void OnCollisionEnter (Collision col)
{
colorChange.material.color = Color.red;
Debug.Log(“Collision Detected”);
}

That should work, if it doesn’t then it may be something within your scene itself that is blocking the collision from happening. The if statement with the tag just allows you to narrow things down to a specific object with that tag.