Hue Online
Hue
A standalone colour picker, converter, search and manipulation tool. A powerhouse of colour functionality for Windows, Linux and macOS.
Download Hue - Linux all distros
Download Hue - macOS 11 Big Sur onwards
Run a virus check on it before you run it. That is for your peace of mind, not mine.
On Windows, if you see "Windows protected your PC", right click the file, select Properties, tick Unblock and click Apply. Then launch the program again.
On Linux, make the file executable first: right click it, select Properties, then Permissions, and tick Allow executing file as program. Then launch the program.
On a Mac, if it will not open, right click the file, select Open, then click Open. On macOS 15 Sequoia and later that no longer works: try to open it once, then go to System Settings, Privacy & Security, and click Open Anyway. Then launch the program again.