Workaround: Manipulate the clipboard content in CommandExecuting even handler:

private void RichTextBox_CommandExecuting(object sender, Telerik.Windows.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is PasteCommand)
    {
        DocumentFragment document = ClipboardEx.GetDocument();
        bool hasChangedDocument = false;

        foreach (FieldRangeStart fieldStart in document.EnumerateChildrenOfType<FieldRangeStart>())
        {
            foreach (Span span in fieldStart.Field.GetFragmentByDisplayMode(FieldDisplayMode.Code).EnumerateChildrenOfType<Span>())
            {
                span.ForeColor = Colors.Black;
                hasChangedDocument = true;
            }
        }

        if (hasChangedDocument)
        {
            ClipboardEx.SetDocument(document);
        }
    }
}