If you keep your phone/tablet on while Youre playing My colony bur youre afk for a long time and not doing anything is that considered cheating?
Nope. Just make manual backups of your game in case something happens to your colony while you are AFK.
But if you run out of food/water/whatever and you revert to an old backup, don't wonder: These resources are synced back from the server and you are out of those on your old backup as well. (In online mode only, of course)
It's risky nowadays.