I found a solution to this; thought I would share...
What I was trying to "turn off" was the tap highlight color. It turns out that you can do this by changing a CSS property for the element you want to control. In my case, the element was the entire HTML container that housed my book.
The CSS is:
This make the highlight color transparent and the net result is that you don't see any highlight on the bounding rectangle at all when you touch an object. Works great for my picture book!