You must be calling SetColor on the wrong object. You need to call it on the SimpleLinePlot object the RTSimpleSingleValuePlot is based on,
SimpleLinePlot lineplot2 = new SimpleLinePlot(pTransform2, null, attrib2);
lineplot2.SetFastClipMode(ChartObj.FASTCLIP_X);
rtPlot2 = new RTSimpleSingleValuePlot(pTransform2, lineplot2, NASDAQChannel);
chartVu.AddChartObject(rtPlot2);
// Change the line color
lineplot2.SetColor(Color.Yellow);