If you right click on the graphic you will bring up a floating menu. Select "Customize..." from the menu. In the dialog box that now appears find the curve for which you want to change the color, you will see some JSL, look for Pen Color( -2994485 ); and change the number to your desired color in " " so Pen Color( "blue"); click the apply button and you should have a new line color.
Right click on the line in the legend and pick the desired color. If you do not have a legend displayed, you can right click on the graph and select Customize.