You can not have line breaks in a string, you have nested double quotes, snd you have a function call that makes no sense.
view: function (item, column) {
var str = "<div id='couleur'>" +
"<div style='background-color:" + item.get('color') + "'></div>" +
"</div>";
return str;
}
Most people would use more of a templating type of architecture.
view: function (item, column) {
var str = "<div id='couleur'><div style='background-color:{color}'></div></div>";
return str.replace("{color}", item.get('color'));
}