XPath: How to select a node by its attribute?

I have an XML that goes like this:

<?xml version="1.0" encoding="utf-8" ?>
<colors>
  <color index = "0">#FF0000</color>
  <color index = "1">#FF0200</color>
  <color index = "2">#FF0300</color>
  <color index = "3">#FF0500</color>
  [..]

I'm trying to select a node by its index:

XmlDocument ColorTable = new XmlDocument();
ColorTable.Load(HttpContext.Current.Server.MapPath("~/App_Data/ColorTable.xml"));
int percentage = 2;
string xpath = string.Format(@"//color[index={0}]", percentage.ToString());
//string xpath = string.Format(@"//color[index=""{0}""]", percentage.ToString());
//string xpath = string.Format(@"//color[index='{0}']", percentage.ToString());
var r = ColorTable.SelectSingleNode(xpath).Value;

I tried also the commented versions, but it does not return any result. Any suggestion?