Hi Bruce,

You'll have to split lines that cross the left to right boundary and assign colors per individual line segment. The following code snippet shows how to do that:

 public class NColoredLine
 {
  #region Constructors

  public NColoredLine(float middle, NLineSeries lineSeries, Color leftColor, Color rightColor)
  {
   m_Middle = middle;
   m_MiddleRay = NLineF.FromTwoPoints(new NPointF(middle, -100000.0f), new NPointF(middle, 100000.0f));
   m_LineSeries = lineSeries;
   m_LeftColor = leftColor;
   m_RightColor = rightColor;
  }

  #endregion

  public void AddLineSegment(NPointF a, NPointF b)
  {
   NLineF line = NLineF.FromTwoPoints(a, b);
   int key = m_LineSeries.Values.Count;

   if (a.X <= m_Middle && b.X <= m_Middle)
   {
    // left
    m_LineSeries.Values.Add(a.Y);
    m_LineSeries.XValues.Add(a.X);

    m_LineSeries.Values.Add(b.Y);
    m_LineSeries.XValues.Add(b.X);

    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_LeftColor);
    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_LeftColor);
   }
   else if (a.X >= m_Middle && b.X >= m_Middle)
   {
    // right
    m_LineSeries.Values.Add(a.Y);
    m_LineSeries.XValues.Add(a.X);

    m_LineSeries.Values.Add(b.Y);
    m_LineSeries.XValues.Add(b.X);

    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_RightColor);
    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_RightColor);
   }
   else
   {
    // intersect
    NPointF split;
    line.IntersectWith(m_MiddleRay, out split);

    // left
    m_LineSeries.Values.Add(a.Y);
    m_LineSeries.XValues.Add(a.X);

    m_LineSeries.Values.Add(split.Y);
    m_LineSeries.XValues.Add(split.X);

    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_LeftColor);
    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_LeftColor);

    // right
    m_LineSeries.Values.Add(split.Y);
    m_LineSeries.XValues.Add(split.X);

    m_LineSeries.Values.Add(b.Y);
    m_LineSeries.XValues.Add(b.X);

    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_RightColor);
    m_LineSeries.BorderStyles[key++] = new NStrokeStyle(m_RightColor);

   }

   
  }

  #region Fields

  float m_Middle;
  NLineSeries m_LineSeries;
  Color m_LeftColor;
  Color m_RightColor;
  NLineF m_MiddleRay;

  #endregion
 }
 public partial class Form1 : Form
 {
  public Form1()
  {
   InitializeComponent();
  }

  private void Form1_Load(object sender, EventArgs e)
  {
   NChart chart = nChartControl1.Charts[0];

   NLineSeries line = new NLineSeries();
   line.DataLabelStyle.Visible = false;
   chart.Series.Add(line);

   NColoredLine colorLine = new NColoredLine(0, line, Color.Green, Color.Red);

   line.UseXValues = true;
   
   float yVal= -40;
   float xVal = -40;

   float prevYVal;
   float prevXVal;

   for (int i = 0; i < 10; i++)
   {
    prevYVal = yVal;
    prevXVal = xVal;

    xVal += 10;
    yVal += 10;

    colorLine.AddLineSegment(new NPointF(prevXVal, prevYVal), new NPointF(xVal, yVal));
   }

   for (int i = 0; i < 10; i++)
   {
    prevYVal = yVal;
    prevXVal = xVal;

    xVal -= 10;
    yVal += 10;

    colorLine.AddLineSegment(new NPointF(prevXVal, prevYVal), new NPointF(xVal, yVal));
   }
  }
 }

Hope this helps - let me know if you meet any problems.

Best regards,
Bob