Do you want to know how to change the graph line color?
How you change graph plotting colors depends on what you are graphing. I essentially use a conditional statement or another way to determine which sensor to plot and use the appropriate PaintColor to plot. Changing the PaintColor is easy; figuring out which sensor to plot and when can be complex. Here is a simplification if color determination:
What you need to do in your app depends on what you are plotting and what you have coded so far.
I suggest you use the code shown elsewhere in Trying to create a graph . Use one of the other graphing options; they are easier to implement. 
However, if you are determined and already are plotting multiple curves, just change the paint color of each curve.