How a daily puzzle picks its answer
The answer has to be the same for everyone, impossible to peek at early, and it can never run out. A list of answers eventually fails all three.
Huedle is our daily colour puzzle: one secret colour, six guesses, a clue after each. Most players never wonder where the colour comes from. We had to, before we could build it.
Why not just keep a list?
Plenty of daily games have a list of dated answers, and for a word game that's right: someone has to check every word is fair. For colours a list is just a problem. It runs out unless someone keeps adding to it. It's stored somewhere, and anything stored can leak, which ruins a daily puzzle. And somebody has to look after it forever.
Work it out instead
Huedle's colour isn't stored anywhere. Each day the server mixes today's date with a secret key that only it knows, and gets a result that looks random but is always the same for that date. The colour is read off that.
Everyone gets the same colour on the same day. Nobody can work out tomorrow's without the key. And it can't run out, because there's always another date.
Not every colour makes a good puzzle
Truly random would sometimes give you almost-black, almost-white or a flat grey, and on those you can't really see the hue. So hue can be anything, but saturation and lightness stay away from the extremes. Every answer is a colour whose hue you can actually make out.
Clues that help without giving it away
After each guess you see, for hue, saturation and lightness, whether you're too high or too low and roughly by how much. The percentage is the overall match. Our eyes notice a wrong hue far more than slightly wrong lightness, so hue counts double in it.
Any solve beats any failed game, however close the near-miss. Fewer guesses score more, and landing very close gets a small bonus. That's all worked out on the server: your browser sends the colours you picked, and the server sends back the clues and the score. (Why that matters is in our piece on leaderboard cheating.)
The catch
The key can never change. Change it in the middle of a day and the answer changes under everyone who's already started, and their clues suddenly point at a colour that isn't there any more.