Highlight smallest value in green and highest value in red

You are probably best of adding this as a document level JavaScript function. To do that, search for the "Document JavaScripts" tool in the 'Tools' tab of Adobe Acrobat Pro DC. Note that if you have Adobe Acrobat Reader, you can't add any JavaScript.

Your code is almost right, but you should put the names of your fields in an array. I assumed that your fields were named "Field #1", "Field #2" etc. But your field names do not include any incremental enumeration, thus the loop won't work unless you have an array with the field names that you loop through.

I would recommend putting the following function as a document-level JavaScript:

function calcFields() {
    //make an array of the numbers and the field names that correspond with them
    var valArray = [[], []];
    var total = 0;
    var NamesArr = ["PreisA11, PreisB11", "PreisC11", "PreisD11"];
    for (var i = 0; i < NamesArr.length; i++) {
        var Nm = NamesArr, fld = this.getField(Nm);
        fld.textColor = color.black; //reset all the field colours
        if (fld.value && !isNaN(fld.value)) {
            valArray[0].push(fld.value);
            total += fld.value;
            valArray[1].push(Nm);
        }
    }
    //get the max value and make it red
    var maxVal = Math.max.apply(null, valArray[0]);
    var maxIndex = valArray[0].indexOf(maxVal);
    var maxFld = this.getField(valArray[1][maxIndex]);
    if (maxFld) maxFld.textColor = color.red;
    this.getField("MAX1").value = maxVal;
    //get the min value and make it green
    var minVal = Math.min.apply(null, valArray[0]);
    var minIndex = valArray[0].indexOf(minVal);
    var minFld = this.getField(valArray[1][minIndex]);
    if (minFld) minFld.textColor = color.green;
    this.getField("MIN1").value = minVal;
    //get the average value
    var avgVal = total / valArray[0].length;
    this.getField("AV1").value = avgVal;
};

I only see four field names in your other comments, but you can add as many as you want to the NamesArr.

Then you need the fields to call upon this function as soon as somebody 'commits' the value to the field.

You can do this by running the code below from the JavaScript console in Acrobat (open it with Ctrl+J).

//Add the formula to the four fields. Only run this code once, from the console!
var NamesArr = ["PreisA11, PreisB11", "PreisC11", "PreisD11"];
for (var i = 0; i < NamesArr.length; i++) this.getField(NamesArr).setAction("OnBlur", "calcFields();");