[Solved]-How to change graph colour above and below plotline in Highcharts-Highcharts

You can do this with a combination of the threshold and negativeColor series options.

      series: [{
        name: 'KPN12345',
        data: [
            [1327881600000, 11],
            etc...
        ],
        threshold: 15,
        negativeColor: 'green',
        color: 'red',
        tooltip: {
            valueDecimals: 2
        }
    }]

Fiddle here.