public void DuplicateAndModifyMaterial(Material material)
{
   ElementId appearanceAssetId = material.AppearanceAssetId;

   AppearanceAssetElement assetElem = material.Document.GetElement(appearanceAssetId) as AppearanceAssetElement;

   ElementId duplicateAssetElementId = ElementId.InvalidElementId;

   using (Transaction t = new Transaction(material.Document, "Duplicate Red Carpet Material"))
   {
      t.Start();

      // Duplicate the material
      Material duplicateMaterial = material.Duplicate("Blue Carpet");

      // Duplicate the appearance asset and the asset in it
      AppearanceAssetElement duplicateAssetElement = assetElem.Duplicate("Blue Carpet");

      // Assign the asset element to the material
      duplicateMaterial.AppearanceAssetId = duplicateAssetElement.Id;

      duplicateAssetElementId = duplicateAssetElement.Id;

      t.Commit();
   }

   // Make changes to the duplicate asset
   using (Transaction t2 = new Transaction(material.Document, "Change blue carpet material"))
   {
      t2.Start();

      using (AppearanceAssetEditScope editScope = new AppearanceAssetEditScope(assetElem.Document))
      {
         Asset editableAsset = editScope.Start(duplicateAssetElementId);   // returns an editable copy of the appearance asset

         // Description
         AssetPropertyString descriptionProperty =
            editableAsset.FindByName("description") as AssetPropertyString;
         descriptionProperty.Value = "blue carpet";

         // Diffuse image
         AssetPropertyDoubleArray4d genericDiffuseProperty = editableAsset.FindByName("generic_diffuse") as AssetPropertyDoubleArray4d;

         genericDiffuseProperty.SetValueAsColor(new Color(0x00, 0x00, 0xFF));

         Asset connectedAsset = genericDiffuseProperty.GetSingleConnectedAsset();

         AssetPropertyString bitmapProperty = connectedAsset.FindByName("unifiedbitmap_Bitmap") as AssetPropertyString;

         bitmapProperty.Value = "Finishes.Flooring.Carpet.4.png";

         editScope.Commit(true);
      }

      t2.Commit();
   }
}

Public Sub DuplicateAndModifyMaterial(material As Material)
    Dim appearanceAssetId As ElementId = material.AppearanceAssetId

    Dim assetElem As AppearanceAssetElement = TryCast(material.Document.GetElement(appearanceAssetId), AppearanceAssetElement)

    Dim duplicateAssetElementId As ElementId = ElementId.InvalidElementId

    Using t As New Transaction(material.Document, "Duplicate Red Carpet Material")
        t.Start()

        ' Duplicate the material
        Dim duplicateMaterial As Material = material.Duplicate("Blue Carpet")

        ' Duplicate the appearance asset and the asset in it
        Dim duplicateAssetElement As AppearanceAssetElement = assetElem.Duplicate("Blue Carpet")

        ' Assign the asset element to the material
        duplicateMaterial.AppearanceAssetId = duplicateAssetElement.Id

        duplicateAssetElementId = duplicateAssetElement.Id

        t.Commit()
    End Using

    ' Make changes to the duplicate asset
    Using t2 As New Transaction(material.Document, "Change blue carpet material")
        t2.Start()

        Using editScope As New AppearanceAssetEditScope(assetElem.Document)
            Dim editableAsset As Asset = editScope.Start(duplicateAssetElementId)
       ' returns an editable copy of the appearance asset
       Dim descriptionProperty As AssetPropertyString = TryCast(editableAsset.FindByName("description"), AssetPropertyString)
       descriptionProperty.Value = "blue carpet"

       ' Diffuse image
       Dim genericDiffuseProperty As AssetPropertyDoubleArray4d = TryCast(editableAsset.FindByName("generic_diffuse"), AssetPropertyDoubleArray4d)

       genericDiffuseProperty.SetValueAsColor(New Color(&H0, &H0, &HFF))

            Dim connectedAsset As Asset = genericDiffuseProperty.GetSingleConnectedAsset()

       Dim bitmapProperty As AssetPropertyString = TryCast(connectedAsset.FindByName("unifiedbitmap_Bitmap"), AssetPropertyString)

       bitmapProperty.Value = "Finishes.Flooring.Carpet.4.png"

            editScope.Commit(True)
        End Using

        t2.Commit()
    End Using
End Sub