You can use the 'Control Graph' method for this.
For eg: (Taking Value as the Measure you have on your line graph and Threshold Limit as the threshold you want the colour of the line graph to be based on)
IF SUM([Value]) > [Threshold Limit]
THEN "Out of Control"
ELSE "In Control"
You can assign colours to 'Out of Control' and 'In Control' in the Colour tab on the Marks shelf to suit your project theme.
The answer given by Jacquelline is the best way of achieving this.
If you need further help please share a workbook and we can demonstrate.