You need to reset color
var to #
again before k
loop
color = '#';
for (var k = 0; k < 6; k++)
....
Also you need to declare idx
before using it
var idx = 0;
for(var j = 0; j < 8; j++)
....
EDIT
Implementing bg color change on button click
Add this code after all finished
var buttons = Array.prototype.slice.call(tbl.getElementsByTagName("button"));
buttons.forEach(function(btn){
btn.addEventListener("click",function(){
document.body.style.backgroundColor = btn.style.backgroundColor;
});
});