Rich text in new UI?

I hope this helps. This works like your Karaoke example. It will temporarily "bold" and change the color of each word in a string from beginning to end with some delay between each word. It works by using StringBuilder.Insert to insert rich text opening tags before the word and rich text closing tags after the word. After this is done, the UI text is updated by setting it equal to the StringBuilder.ToString(). The tags are then removed using StringBuilder.Remove in preperation for the next word.

I don't know if this is the official way of doing this sort of thing, but it does seem to work. It wouldn't be too hard to use StringBuilder.Replace to make it work on specific words etc..

 sBuilder.Replace("words", "<b><color=red>words</color></b>");

Unity Rich Text Manual Page

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using System.Text;
 
 public class RTTest : MonoBehaviour
 {
     string boldText = "Those are some bold words!";
     private Text textToDisplay;
 
     void Start()
     {
         textToDisplay = GameObject.Find("RichTextTest").GetComponent<Text>();
         StartCoroutine(BoldWords(boldText));
     }
 
     IEnumerator BoldWords(string textToBold)
     {
         string rtOpenTag = "<b><color=green>";   //Rich text opening tag
         string rtCloseTag = "</color></b>"; //Rich text closing tag
         int wordStartIndex = 0;       //Start index of a word
         int wordEndIndex = 0;         //End Index of a word
         float delay = 1.0f;
         StringBuilder sBuilder = new StringBuilder(textToBold);
 
         while (wordEndIndex < textToBold.Length)
         {            
             wordStartIndex = GetNextStartIndex(textToBold, wordEndIndex);    
             wordEndIndex = GetNextEndIndex(textToBold, wordStartIndex);
             sBuilder.Insert(wordStartIndex, rtOpenTag);                
             sBuilder.Insert(wordEndIndex + rtOpenTag.Length, rtCloseTag);
             textToDisplay.text = sBuilder.ToString();                  
             sBuilder.Remove(wordStartIndex, rtOpenTag.Length);            
             sBuilder.Remove(wordEndIndex, rtCloseTag.Length);                             
             yield return new WaitForSeconds(delay);                      
         }
         textToDisplay.text = sBuilder.ToString(); 
     }
 
     int GetNextStartIndex(string textToBold, int wordEndIndex)
     {
         int nextStartIndex = wordEndIndex;
 
         if (textToBold[nextStartIndex] == ' ')
         {
             while (textToBold[++nextStartIndex] == ' '){ }
         }
         return nextStartIndex;
     }
 
     int GetNextEndIndex(string textToBold, int wordStartIndex)
     {
         int nextEndIndex = wordStartIndex;
 
         while (++nextEndIndex < textToBold.Length && textToBold[nextEndIndex] != ' ') { }
         return nextEndIndex;
     }
 }