You can set the LineColor property of the LineObject class.

Get a reference to the object using the ReportDocument class.

(CrystalDecisions.CrystalReports.Engine.LineObject)ReportDocument.ReportDefinition.ReportObjects['line object name'].LineColor = Color.Red;

Replace 'ReportDocument' with your report object name. I have the complete CR.NET object model mapped out in my books. You can find out more about my books at Amazon.com or reading the Crystal Reports eBooks online.