A-Frame tutorials use "document.querySelector"; however, using document.querySelectorAll, has a bug that doesn't apply. Also if you want to affect newly added objects, querySelector was having issues (Edit: It is working now).
Luckily, we have a workaround to use jQuery.
$("body").on("click","a-entity",function(){
console.log("Clicked!");
this.setAttribute('material', 'color', 'red');
});
console.log("Clicked!");
this.setAttribute('material', 'color', 'red');
});
With the above code you can click on any object as long as it is in the cross hairs.