What is the JavaScript for turning off the "Highlight Colour" on a text box once you have filled it in - Adobe Form?

You'll first need to disable the normal runtime highlight in a document level script using...

app.runtimeHighlight = false;

Then in the format script of each field, add the following...

var field = event.target;
if (field.value == field.defaultValue) {
     field.fillColor = color.cyan; // for example
}
else {
     field.fillColor = color.transparent;
}

The format script runs last so that's the place to put the code. In this case, you're formatting the appearance of the field rather than formatting the value though.