To paint the background of the graph white temporarily turn off the X and Y grid and use the paint tool. From the question I understand you've managed that part, right?
To change the signal color just click on the graph legend (if it's not showing right-click on the graph and choose to show it) with the finger tool and you will get a menu where you can do a number of things with the signal line including changing the color.
Or just paint the signal line shown in the legend.
You can off course also do all this programmatically by creating a property node for the graph and wire color constants to the color properties.