The phrase "purple line" typically means a boundary or limit that should not be crossed. It refers to a line that separates what is acceptable or allowed from what is not. Full definition
It uses purple lines on the touchscreen to show where the trailer will go, and these lines turn green when the trailer has reached its proper position. (landroverftmyers.com)
Here you can see that one teacher, represented by the dark purple line, is recording behaviors significantly more than the rest of the selected teachers. (kickboardforschools.com)
One morning I got the joyful sign: a faint purple line. (onbeingmom.com)