Simply keeping your feet warm may help fend off colds:

But when he and a colleague deliberately chilled subjects — by dipping their feet in ice water — they found that those who got their feet cold and wet were significantly more likely to develop symptoms of a cold over the next four or five days than were those who just put their feet in an empty bowl.

While this study did not look at this it seems reasonable to conclude that folks with circulatory problems that lead to cold extremeties should, if the study is correct, show a higher incidence of colds.
Investors might note an uptick in sales of foot warming devices if this study gets much play in the popular press.