Economic factors affecting participation
If your best place to site your gallery is near your home, but your home is in a lower-income neighborhood, potential participants may be stopped just because they don’t have art supplies at home. This is a case where it makes sense to have an art supply shelf in your gallery, with papers and canvases precut to appropriate sizes and signage encouraging people to make something to bring back. Better still, coordinate with art teachers in local public schools to encourage the kids to create art for the gallery in class (you might still need to donate some supplies). Or, space permitting, set up a tent and tables and have regular art events for the neighborhood, preferably with a theme — love in February, spooks in October, etcetera.
If you just set out supplies, you might find that people take them but don’t return a lot of artwork, which is disappointing. But if they’re taking them to do art, that’s still a good thing to encourage. If you have strategies that work for getting people to return with their finished work, please get in touch!
Too much turnover in visitors
I haven’t come across anyone actually trying this, but if you do, please let me know so I can find out how it works. If your only good place to set up a gallery is in a commercial area where there’s not a lot of repeat visitors, think about ways to get them to create art while they’re still in the area. You could, for instance, coordinate with the restaurant across the road to have supplies available and signage encouraging people to use the colored pencils to draw while they wait for their lunch. If you’re near a B&B or apartment share, likewise, try to get the owner to put in signs and supplies. Talk with the librarians at your local public library and see whether you can arrange an art table there that they collect the work from, for you to pick up periodically. Use your imagination.