Setting field color

Good morning,

I am attempting to set a field color based on contents of some other fields;

I referred to : Change Field Color in MS CRM 365 (msdynamiccrmcustomizations.blogspot.com)

The field I am looking at is in a Tab called: Order Product Families (Label is the same).

The field is: sop_australianshutterduedate

Note that I am happy to set either/both the label and field to say Green depending on conditions:

My Javascript onLoad first few lines look like this;

   var formContext=executionContext.getFormContext();

Then I do the first step (from attached article)

 var scandoc = parent.document.getElementById("sop_australianshutterduedate");

and that is as far as I can get - because scandoc is returned as null..................

Can anyone advise - because if I cannot get past this first step, obviously I cannot set the color...........

All the best

Stan