i think anybody with normal eyesight could spot a lurcher at a good distance what ever colour it was, i know i could.outlines, shapes, and movement register imediatley with anyone and if someone is watching or looking for someone with a dog the chances are they will spot what they are looking for anyhow.
how often do we spot deer, hares foxes etc at great distances ? all of which are dark in colour.
i think it could be a slight disadvantage (if you dont want to be seen) having a white dog when out at night, but as i said earlier if your out with the lurcher at night your probably lamping and