Hi stuartdj,
I'm not finding a way to highlight text as one can do with iBooks (hold and swipe for black text, background yellow for example).
One option is to actually color the text itself.
Select the text, then in Style tools, tap the Font name at the top which shows the Text Options sheet where there is a Color option. Would changing the font color to Red, Green or Purple accomplish your goals?
If you do want black text on a colored background (highlighted) it would need to be text in a text box where you can change the fill (background) color. Style, Style Options, Fill. But this is not inline text and doesn't flow like just coloring the font itself.
ivan