How to make a colored square with string builder

Hi, I need to draw a colored square inside a string
the followng solution does not work
sb.AppendFormat(@"<div class='color-box' style='background-color: #FF850A;'></div>"); string Name= sb.ToString() + r.Name; but this works
sb.AppendFormat(@"<b>ok</b>"); Name = sb.ToString() + r.Name;