public class CarService : System.Web.Services.WebService
{
    private static XmlDocument doc = null;

    public XmlDocument Doc
    {
        get
        {
            if (doc == null) // Read XML data from disk
            {
                doc = new XmlDocument();
                doc.Load(HttpContext.Current.Server.MapPath("~/App_Data/Cars.xml"));
            }
            return doc;
        }
    }

    private List<string> getNames(string path)
    {
        List<string> nameList = new List<string>();
        try
        {
            XmlNodeList nodeList = Doc.SelectNodes(path);
            foreach (XmlNode node in nodeList)
                nameList.Add(node.Attributes["name"].InnerText);
        }
        catch (Exception ex)
        {
            nameList.Add(ex.Message);
        }
        return nameList;

    }
/*
    [WebMethod]
    public List<string> GetMakes()
    {

    }

    [WebMethod]
    public List<string> GetModels(string make)
    {

    }
*/
    [WebMethod]
    public List<string> GetColors(string make, string model)
    {
        string path = String.Format("/cars/make[@name='{0}']/model[@name='{1}']/colors/color", make, model);
        return getNames(path);
    }
}