Research version. It keeps the three methods, the hue sweep and the real-theme check. The simpler picker is index.html.
Colours for the visitor site's theme, in two parts. Presets: ready-made light and dark sets to pick from. From one colour: type or pick a single brand colour and get all six colours (primary, page and card, for light and dark) back, with three different methods side by side. Every tile is drawn with the rules of the Preview on the admin Site > Theme page.
Edit any of the six to adjust by hand; the tiles and the checks follow.
Each method gets the same brand colour. Rules is the hand-written OKLCH logic from this project; Material 3 is Google's tonal-palette algorithm; Ant Design is the generator of that design system. Page and card style applies to Rules only: the other two bring their own greys.
The same lightness and vividness as the colour above, twelve hues, through the selected method. Click one to use it. Yellow, green and cyan cannot be as vivid as blue or purple at the same lightness; where the colour turns dull, that is the colour space, not a bug.
Take a real theme, give the method only its light primary, and compare what comes back with the theme's own five other colours. A distance below 0.02 is barely visible, above 0.05 is clearly different.