Вы можете попробовать с XDocument
XDocument xdoc = XDocument.Load("XMLFile7.xml");
var mdAarray = xdoc.Descendants("Etiquette")
.SelectMany(etiquette =>
etiquette.Descendants("string")
.Select(associatedaffaire => new string[] {
etiquette.Element("BgColor").Value.ToString(),
etiquette.Element("BorderColor").Value.ToString(),
associatedaffaire.Value.ToString(),
etiquette.Element("Label").Value.ToString() }))
.ToArray();
Console.WriteLine(JsonConvert.SerializeObject(mdAarray));
ВЫХОД
[
["#8075D1C5","#FF4E5B6F","d4689f33-5600-47fe-883d-efcbf5e469c2","Ouverte"],
["#8075D1C5","#FF4E5B6F","1bae35dd-d501-4d87-bdd4-147fc0ba29d2","Ouverte"],
["#80949CA8","#FF155E70","203cc4a8-8c24-4a2d-837c-29c7c1f73007","Fermée"]
]