"Even with the ever-increasing prominence of iPhones and Androids, it is exceedingly unlikely that you'll be at a location where multiple people own those phone

"Even with the ever-increasing prominence of iPhones and Androids, it is exceedingly unlikely that you'll be at a location where multiple people own those phones, have downloaded color, and are using the app at that precise moment" The only hard part is getting people to install it. Once it is installed push notifications or constant polling in the background can tell you when others nearby are using it -- I imagine a combination of fine GPS, WiFi SSID comparisons, and cell tower IDs would fine tune it. Of course, not being a mobile developer I don't know whether the last two are even obtainable by a third-party app.