In order to change the colour and size of value in bar charts you can use the following script:
widget.on('processresult',function(w,e){
e.result.plotOptions.series.dataLabels.style.color = "green";
e.result.plotOptions.series.dataLabels.style.fontSize = "19px";
e.result.plotOptions.series.dataLabels.style.fontWeight = "bold";
})
