This page illustrates a number of Purple Sandpipers photographed in northeastern Scotland in late February 2009. I have concentrated on aspects that may help in evaluating differences between this taxon and Rock Sandpiper.
For comparison, here are two links to photos of Rock Sandpipers of the mainland western Alaska breeding population tschuktschorum - this is the subspecies that winters well to the south to northern California, and thus is the most likely form to be a potential vagrant in North America.
http://www.birdingak.com/2010/12/02/wintering-rock-sandpipers-in-homer/ - a blog page with numerous photos of this ssp. plus a presumed nominate ptilocnemis mixed in with them.
http://www.birdingak.com/wp-content/uploads/2010/12/ROSA-wing-patterns.pdf - this page is linked-to within the above web page, and shows a sample wing for each of the 3 ssp. found in North America.
These two are the same photo, the first with no adjustment, the 2nd with some mid-tone adjustment to counter the over-exposure of the underwing:
These images were selected to show variation in bare parts colour compared to a control species - Ruddy Turnstone:
A potential fly in the ointment: it seems that some Purple Sandpipers can have fairly obvious blackish smudges on the underparts, as evidenced by this photo of birds selected from
c.100 specimens at the Field Museum of Natural History, Chicago: