This idea of naval flags and codes and stuff. As Pedro said most people wont know it. Colored smoke, time color changing paint, or any other code is worthless unless comunicating with people you know or or to naval vessals.
Now Pedros idea of using known flags or symbals to indicate safety, danger, or what ever could be useful. IE red cross for medical.
Now the issue of a settlement flag. If they can see your settlement flag they will see your site already. Unless your like in the forrest or you plant it on a hill or such. So its seen and not the settlement. Its a risk and if your strong enough then go for it. You will have to name the settlement anyways so people know what to call it when telling others.
This false hope you will be the only group for 100s of mile is more fantasy than a Zed Apox. If one in 10,000 survive, do the math there will still be thousands of survivors in most places just from the population sizes.
As for colored smoke, and other ideas. Save your effort. Put a short flag out a window or on the roof. Raise it and lower it everyday an rotate between multiple flags. IE United States, Ohio, college team, ect. Just be creative, so if they come say on Monday then return on say Friday they will notice a new flag. You could even rotate flags through the day. Its up to you.
People will be always on the move after an Apox. Some to find lost family, safer area away from raiders or just the group they were with. And some will restart the old nomad traditions because they dont want to settle down.
There will be scouts looking for other settlements or raider camps. Intel of all sorts is needed at all times and a flag says we are here come visit. It also tells Raiders your there but if you think you can hide a colony/settlement and the noises it makes from raiders you are in fantasy land.
So just face up to the fact and make a statement if you want and how you want it wont hurt a bit.

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