iOS gives no app permission to change your screen colour. Only a shortcut you build yourself can. Open Doonce, tap HELP ME SET THIS UP, and follow the eight panels. The names have to be exactly Doonce On and Doonce Off, because that is what Doonce looks for.
No. Doonce never blocks itself, so the off switch is always reachable, and GO SMART ends a session immediately. There is no timer you cannot cancel.
Apple only lets an app be excepted from the block one app at a time. A whole category cannot be kept open, so Doonce only counts individual apps.
No. Apple hands the app an anonymous token, never a name or an icon it can read. Nothing is stored anywhere but your own phone, and Doonce has no servers at all.
Blocking stops an app being opened. It does not stop its notifications, and no app on iOS can. That is what the SILENCE half of the setup is for: your shortcut sets a Focus.
Open Doonce and tap a square in the row to reopen Apple's picker. Anything ticked there stays reachable the next time you press GO DOONCE.
Deleting Doonce removes every block immediately. iOS lifts all restrictions the moment an app's Screen Time permission goes away.