I don't think it really matters as long as you keep the phase correct.
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Tpreaves answer is correct. Since purple does have some red in it, (red and blue make purple)you could use the purple as red if that helps you keep it straight. You certainly could use the green as red too, as long as the + and - are connected with the same color at both ends, unless you WISH to invert polarity.
@Tpreaves- thanks some friends who are not too particular tried that.
@Jmcgrogan2- yup purple does have red. Came across this on the XLO website...
According to the XLO website, "The lighter color lead Is attached to the positive terminal".
So as far as the Reference line is concerned, purple is positive.