Jason,

This is exactly what I was look for.  Thanks for the thorough explanation and advice on more elegant code!  I do have a couple of problems though:

1.  Target.addClass('ratingError'); isn't doing anything.  I added that to the CSS section and looked for typos, but can't figure it out.

2.  If I manually add that class to a field it makes the background of the label red, instead of just the background of the input field.

If you're getting the label as red, then you actually added the class to the wrapper around the field rather than the input element.

Each item on a Laserfiche form is encased in a div element that holds the label and input; this div element is the one that actually gets the class assigned in the advanced tab.

However, the method I provided should be assigning the "ratingError" class directly to the input element.

Can you repost with your updated code so I can take a look? Also, inspect the page to look for JavaScript errors, and to determine whether the class is not getting added at all or if it just isn't affecting the style.

Here is the code I added

$(document).ready(function(){
    // You can assign the change handler directly to the element
    // This is much easier to read/manage than using $(document).on('change') and specifying a target
    $('.InsResistTest3 input').change(function(){
      // Call the generic function and pass in the objects that will be compared/updated
      // Passing in your PFField, VField, and object that triggered the event
      rateComparison($('#q345 input'),$('#q309 input'),$(this));
    });
});

function rateComparison(PFField, VField, Target){
    // Get your rating and values from the inputs
    var VRating = parseFloat(VField.val().replace(/,/g,''));
    // Target is your $(this) object passed in from the calling event handler
    // Getting the value and storing it to a variable is more efficient than calling $(this).val() over and over
    var TValue = Target.val();
  alert(Target);
  alert('before if'); 
  
    // Use the OR || operator so any of the conditions will trigger the associated changes
    // All of your IFs have the same result so there's no real reason to split them up
    if ((VRating <= 250 && TValue < 0.025) || 
        (VRating > 250 && VRating <= 1000 && TValue < 0.100) ||
        (VRating > 1000 && VRating <= 2500 && TValue < 0.500) ||
        (VRating > 2500 && VRating <= 5000 && TValue < 1.000) ||
        (VRating > 5000 && VRating <= 8000 && TValue < 2.000) ||
        (VRating > 8000 && VRating <= 15000 && TValue < 5.000) ||
        (VRating > 15000 && VRating <= 25000 && TValue < 20.000)){
            // Instead of changing the style just add class reference
            // This will give you more control and make maintenance easier
            // because you can edit the CSS instead of the JavaScript
            
      		Target.addClass('red');
      		//this.setAttribute('style', 'background-color: tomato !important');
            PFField.val('F').change();
    }
    else {
      // Remove the custom class so the default color will show and it will
      // match the other fields even if you make any adjustments to the themes
      alert('else');
      Target.removeClass('red');
      PFField.val('PASS').change();
    }
}

and my css

/*Makes background red to indicate a failed test result, called by javascript */
.red {
  background-color:tomato !important;
}

It isn't working because the field I am trying to 'highlight' tomato is read-only.  Do you know how to work around that?

Sorry, I'm a bit confused. The way the code is written the field it is trying to highlight is the field with the event handler.

If the field is read-only, what is triggering the change event?

In either case, if it is read-only you'll have to increase the specificity of the CSS selector to make sure it overrides the default styling.

Try this

.red,
input[backend-readonly].red {
  background-color:tomato !important;
}

If the field is read-only you don't technically need the ".red," portion, but I left it in for comparison.

The field has a formula applied to it.  The formula takes the value from the field above it and runs a calculation to temperature adjust it.  I think that answers your question?  I am learning as I go here.

If I try to use 

Target.setAttribute('style', 'background-color: tomato !important');

in place of 

Target.addClass('failedTest');

The console (in Chrome) shows Target.setAttribute is not a function