rtb.Selection.Foreground = newSolidColorBrush(Colors.Red);
Highlight (background) color
rtb.Selection.InlineBackground = newSolidColorBrush(Colors.Yellow);
Toggle Bold
if(rtb.Selection.FontWeight != null&& rtb.Selection.FontWeight.Value.Weight == FontWeights.Bold.Weight)
{
    rtb.Selection.FontWeight = FontWeights.Normal;
}
else
{
    rtb.Selection.FontWeight = FontWeights.Bold;
}

if(rtb.Selection.FontStyle != null&& rtb.Selection.FontStyle == FontStyle.Italic)
{
    rtb.Selection.FontStyle = FontStyle.Normal;
}
else
{
    rtb.Selection.FontStyle = FontStyle.Italic;
}

var range = rtb.Selection;
var collection = new C1TextDecorationCollection();
if (range.TextDecorations == null)
{
    collection.Add(C1TextDecorations.Underline[0]);
}
else if (!range.TextDecorations.Contains(C1TextDecorations.Underline[0]))
{
    foreach (var decoration in range.TextDecorations)
        collection.Add(decoration);

    collection.Add(C1TextDecorations.Underline[0]);
}
else
{
    foreach (var decoration in range.TextDecorations)
        collection.Add(decoration);

    collection.Remove(C1TextDecorations.Underline[0]);
    if (collection.Count == 0)
        collection = null;
}
range.TextDecorations = collection;

rtb.Selection.InlineBackground = null;
rtb.Selection.Foreground = rtb.Foreground;
rtb.Selection.FontWeight = FontWeights.Normal;
rtb.Selection.FontStyle = FontStyle.Normal;
rtb.Selection.TextDecorations = null;

private void RichTextBoxSelectionChanged(object sender, EventArgs e)
{ 
    var sel = _rtb.Selection;

    FontWeight? fw = _rtb.Selection.FontWeight;
    _btnBold.IsChecked = fw.HasValue && fw.Value == FontWeights.Bold;

    FontStyle? fs = _rtb.Selection.FontStyle;
    _btnItalic.IsChecked = fs.HasValue && fs.Value == FontStyles.Italic;

    _btnUnderline.IsChecked = (sel.TextDecorations != null);
}

private void UnderlineClick(object sender, RoutedEventArgs e)
{
    if (_btnUnderline.IsChecked.HasValue)
    {
        _rtb.Selection.TextDecorations = _btnUnderline.IsChecked.Value
        ? C1.WPF.RichTextBox.Documents.C1TextDecorations.Underline
       : null;
    }
}

private void ItalicClick(object sender, RoutedEventArgs e)
{
    FontStyle? fs = _rtb.Selection.FontStyle;
    _rtb.Selection.FontStyle = fs.HasValue && fs.Value == FontStyles.Italic
        ? FontStyles.Normal
        : FontStyles.Italic;
}

private void BoldClick(object sender, RoutedEventArgs e)
{
    FontWeight? fw = _rtb.Selection.FontWeight;
    _rtb.Selection.FontWeight = fw.HasValue && fw.Value == FontWeights.Bold
        ? FontWeights.Normal
        : FontWeights.Bold;