title_body: Efficient way to colour code text?
upvoted_answer: If you want to stick with EPUB, you can save space by giving each tone a class with a short name and defining the class's color in the CSS. So the example you gave would look something like this:
  
  &lt;span class="a"&gt; 今 &lt;/span&gt;
  
  
  The CSS file, named something like stylesheet.css, would define the class like this:
  
  span.a {
      color: #64B4FF
  }
  
  
  You'd need to add a link to the stylesheet in the head element of each HTML file like this:
  
  &lt;link href="stylesheet.css" type="text/css" rel="stylesheet"/&gt;
  
  
  The path in href is relative, and this example assumes stylesheet.css is in the same directory as the HTML file.
  
  I don't know if this approach will save enough space to speed up highlighting and dictionary lookups, but it's something to try.