Hello Raja K!
It is not so easy to create the color variation above and below the 'broken' threshold line. You should find the intersection points between the series and then use
zones to add a proper color to part of the series, according to the logic of finding intersection points you should use a different
zoneAxis. You can see it in the demo below. It works well with
type: 'line', but it is problematic with splines because mathematics is much more complicated. You can try to implement a function to calculate intersection points for spline type. Here is a similar topic:
viewtopic.php?t=42935
Demo: https://jsfiddle.net/BlackLabel/a4hLqywt/
API Reference: https://api.highcharts.com/highcharts/series.line.zones
https://api.highcharts.com/highcharts/s ... e.zoneAxis
Regards!