Does color by number work without internet?
A colouring app has no honest reason to talk to a server. What that costs, and what it buys, stated in both directions.
A colouring app is one of the few things on a phone with no inherent reason to talk to a server, which makes it a useful place to ask what the network is being used for.
What a colouring app would need a connection for
Downloading the artwork
Pictures are the large objects here. Keeping the catalogue on a server means the app installs small and each picture is fetched the first time you open it. That is a real advantage: a small download, and a catalogue that can grow after you have installed the app.
It also means the app has a dependency it did not need to have. On a plane, on an underground train, in a hotel with a captive portal, or on a patchy connection, opening a picture becomes something that can fail. And a picture can be withdrawn from under a half-finished session.
Accounts and sync
An account exists so that progress can follow you between devices and survive a reinstall. That is a genuine benefit and not a trick. It requires three things, though: an identity for you, a server holding your data, and authentication wrapped around both.
Advertising and analytics
Ad requests are network calls by definition — an advert has to be fetched before it can be shown. Analytics send a continuous stream of events describing what you tapped and when. Both produce background traffic, and neither is anything the colouring itself needs.
What happens here instead
Every page Numbrush ships with is inside the app. The pages-CDN address defaults to an empty string, so no fetch is ever attempted. This is a different thing from an app that caches for offline use: there is no request that could fail, because there is no request.
There is no account. The user id is an anonymous UUID generated on the device on first run and kept in the device keychain. There is no authentication anywhere in the app — nothing to sign into, nothing to sign out of, no password to reset, no email to give.
Progress is a local record: one byte per region, stored as a small array in a table on the device. The picture you see is not stored anywhere — it is regenerated from the read-only region-id map and that array of bytes, every frame. The whole saved state of a picture is a few hundred bytes.
That size is worth noticing, because it makes the absence of sync a decision rather than a technical limitation. A few hundred bytes is nothing to transmit. What transmitting it would require is the account, the server and the identity, and it is those that were left out.
The practical test is the simple one: turn on airplane mode and nothing about the app behaves differently.
The costs, which are real
Install size
Bundling the artwork means paying for every picture on disk whether or not you colour it. A streaming catalogue installs smaller. What the trade buys is a guarantee instead of a probability — no picture can fail to load, no picture can disappear, nothing depends on signal — and guarantees are worth more than they look on a train.
Nothing follows you to a second device
This is the honest price of having no account. A picture half-finished on a phone does not appear on a tablet. Two devices are two separate libraries with no relationship to each other, and there is no mechanism by which they could learn about one another, because neither knows who you are.
There is no backup on any server
Progress lives in the device’s own storage. Delete the app, or lose the phone, and it goes. A device backup that includes app data will carry it across to a new phone; nothing else will. No support request can restore it, because there is no copy on a server to restore from and no way to establish which anonymous id was yours.
The symmetry is exact, and it is the point. The property that makes recovery impossible is the same property that means nothing about you is held anywhere. You cannot have the recovery without the account, and the account is the thing that would have to know who you are.
Which side of that trade is right depends on what you are storing. For a document you have worked on for a week, sync is obviously worth an account. For a half-coloured mandala, the calculation is less obvious, and it is reasonable to decide that a picture is not worth a login.
Frequently asked questions
Does Numbrush work in airplane mode?
Yes, identically. Every page it ships with is inside the app and the pages-CDN address defaults to empty, so no network request is ever made. This is not caching for offline use — there is nothing that could fail to download.
Do I need an account to colour?
There is no account to make. The user id is an anonymous UUID generated on your device on first run and kept in the device keychain, and there is no authentication anywhere in the app.
Will my progress move to a new phone?
Only through a device backup that includes app data. There is no server-side copy and no sync, so a second device starts fresh. That is the direct cost of having no account, and it is worth knowing before you start a large picture.
Why not sync progress if it is only a few hundred bytes?
Size is not the obstacle. Syncing would need an identity for you, a server holding your data and authentication around it — the whole apparatus of an account. The bytes are trivial; the apparatus is the part that was deliberately left out.
Try it on a real canvas
Numbrush is colour by number for adults: mandalas, rose windows and geometric pattern work, twelve colours a picture, a magnifier for the tight spots, and a wrong tap that costs you nothing. Free, offline, no ads and no account.