Checking a sealed colour
The colour is drawn before your choice, and sealed. Here is how to check it yourself, without asking the Centre for anything.
The calculation runs in your browser · it goes nowhere
In five sentences
Before each attempt, the server draws a colour and a thirty-two-byte secret, forms the string made of the colour’s French name in lower case, a colon and the secret, and computes its SHA-256 fingerprint.
That fingerprint is sent to the page before you choose, and it teaches nothing about the colour: a SHA-256 fingerprint cannot be run backwards. It is no longer displayed during the series — sixty-four characters at every attempt taught nobody anything — but it is kept, attempt by attempt.
After your choice, the server reveals the colour and the secret: you then hold the whole string. Both wait for you in your logbook, inside The Experience, under “The sealed attempts of this series”.
Recompute its fingerprint — here, or with any tool you like — and compare it with the one that had been announced.
If the two match, the colour was fixed before your choice: the server could no longer change it without the calculation giving it away.
The calculation, here
Fingerprint computed:
The Prediction, sealed the same way
The point of The Prediction is drawn each day at twelve twelve, never before. The server forms the string made of the two coordinates separated by a comma, a colon and the secret — for instance 481,207:9f3a… — and publishes its SHA-256 fingerprint at the instant of the draw. Coordinates and secret are revealed in the same movement: the tool above applies to them as it does to the colours.
The same calculation, elsewhere
Nothing obliges you to take our word for it, and that is precisely the point. On a Mac or under Linux, the command printf 'vert:9f3a…' | shasum -a 256 gives the same fingerprint; under Windows, certutil -hashfile or any SHA-256 calculator will do. Take care not to add a line break: printf adds none, echo adds one, and the fingerprint would differ.
At the end of a series, The Experience shows the twenty attempts side by side: the fingerprint announced before each choice, then the colour and the secret revealed after. Your logbook reopens them at any time, series by series.