Every measured colour on the property, 277 of them, and five slots to put them in. Pick a ground, a dominant, a secondary, an anchor and an accent — the page checks the same things the documents check, while you work.
What it will not let you do. There is no colour wheel and no free hex entry: every colour here is a real reading off a real object, and that is the whole basis of this system. If you pick two colours a person could not tell apart it says so immediately, using the same threshold the documents use. It also tells you whether what you have chosen could be built from things that move, or whether it commits you to buying.
What it will help you do. Ask it what goes with a colour and it answers from the corpus rather than from theory — the nearest true complement that actually exists on this property, which is usually not the one a hue wheel would suggest. Save as many palettes as you like; they live in this browser. The whole state is in the address bar, so a palette can be sent to somebody by pasting the link.
Click a colour to drop it into the next empty slot. Hold and it tells you what it came off.
Everything here is a measured reading. The same rules the printed documents follow are enforced live: nothing invented, no two indistinguishable colours side by side, roles always explicit, contrast computed rather than assumed.