If we have a paragraph containing single span which has set red foreground as a local value, applying 'Heading 1' style on that paragraph from the RadRichTextBox' Styles Gallery doesn't change the span's foreground to blue (which comes from the applied style), but preserves the red value.

Workaround: To clear a given property, basing on some logic which determines if it should be cleared. For example:
this.radRichTextBox.CommandExecuting += (s, e) =>
 {
     if (e.Command is ChangeStyleNameCommand)
     {
         var style = this.radRichTextBox.Document.StyleRepository.GetValueOrNull(e.CommandParameter.ToString());
         if (style.Type == Telerik.Windows.Documents.Model.Styles.StyleType.Paragraph)
         {
             Paragraph paragraph = this.radRichTextBox.Document.CaretPosition.GetCurrentParagraphBox().AssociatedParagraph;
             foreach (Span span in paragraph.EnumerateChildrenOfType<Span>())
             {
                 span.ClearValue(Span.ForeColorProperty);
             }
         }
     }
 };