Find the earliest date and highlight green and the latest date in red

I wrote a quick script that does what you want. I am not so experienced with JavaScript either, so there is probably a better way of doing this.

This does assume that:

1. Your field names are "field #1", "field #2", "field #3","field #4", "field #5"
2. The dates in those fields are formated as "dd-mm-yyyy". If this is not true, adjust line 5 accordingly.

//make an array of date values
var dateArray = [[], []];
for (var i = 1; i < 6; i++) {
    var Nm = "field #" + i, fld = this.getField(Nm);
    fld.textColor = color.black; //reset all the field colours
    var valDate = util.scand("dd-mm-yyyy", fld.value); //note that "dd-mm-yyyy" must correspond to how the dates are formatted in your fields
    if (fld.value && valDate) {
        dateArray[0].push(valDate.valueOf());
        dateArray[1].push(Nm);
    }
}
//get the max value
var maxIndex = dateArray[0].indexOf(Math.max.apply(null, dateArray[0]));
var maxFld = this.getField(dateArray[1][maxIndex]);
//get the min value
var minIndex = dateArray[0].indexOf(Math.min.apply(null, dateArray[0]));
var minFld = this.getField(dateArray[1][minIndex]);
//set the colours
maxFld.textColor = color.green;
minFld.textColor = color.red;

You could add this script as the OnBlur event for each field. Or beter yet, add it as a document-level function and call that from each OnBlur event.