Panic's Developer ColorPicker (v1.5.4) does not work in macOS Catalina or macOS BigSur. This is due to enforcement of code-signing. Until Panic releases a newer version that supports Catalina/BigSur, below is the workaround.
You need a Developer Account. This could be a free account. Refer: https://9to5mac.com/2016/03/27/how-to-create-free-apple-developer-account-sideload-apps/