I want to create a free/simple web map where users can click & drag to select polygons in the map, then once selected, choose a color for all selected polygons. The map should also maintain these changes even after the user is done (not just browser-side styling).
For example: A simple web map containing polygons of the 50 US states is created. Dave opens the page, clicks & drags to select Texas and New Mexico, and chooses the color red, which colors both states red at once. Later, John opens the page and sees that Texas & NM are red, and then clicks and drags to select New Mexico, Arizona and California and changes their color to blue at once.
I'm experienced in using various mapping libraries (ArcGIS JS API, Google Maps API, Leaflet, etc), so it doesn't really matter which I use, but it just needs to be as simple as possible and free (e.g. no ArcGIS Server). This isn't a commercial application - purely recreational, but I can't seem to think of a simple solution for this.