When you update any Managed Metadata column values from either CSOM of Full Trust Code, you need to make sure to make the Taxonomy value GUID lower cased. Otherwise it will cause the Office Document Information Panel to display an invalid default value (in red). Example code to update a default value, while ensuring the GUID is lowercased is shown below:

SPField field = SPContext.Current.Web.Fields["CustomMMSfield"];
TaxonomyField taxonomyField = (TaxonomyField)field;
TaxonomyFieldValue defaultValue = new TaxonomyFieldValue(taxonomyField);
defaultValue.PopulateFromLabelGuidPair("Netherlands|17E7FDD7-9AAF-4942-BCE1-16f06E40E085");
defaultValue.WssId = -1;
// GUID should be stored lowercase, otherwise it will not work in Office
defaultValue.TermGuid = defaultValue.TermGuid.ToLower();
// Set the selected default value for the site column
taxonomyField.DefaultValue = defaultValue.ValidatedString;
taxonomyField.Update(true);

Please note that the above code is Full Trust Code. Read this post to know how to set default values from CSOM.