With the macOS Monterey and iPadOS 15 release candidates that were seeded to developers today, Apple has eliminated the option to use the Color Tab Bar with the separate tab design in Safari.
A tab bar with color in Safari 15
If you want to have a color tab bar that blends in with the background color of a website, you need to toggle on the Compact Tab Bar, which is the tab bar design that Apple introduced with macOS Monterey and iPadOS 15 updates. "Show Color in Compact Tab Bar" is an option that can be enabled in both iPadOS 15.1 and macOS Monterey.
On the iPad, the option is available in the Safari section of the Settings app. Scroll down to the bottom to find it and toggle it on, and then the color tab interface will show up when the iPad is using the Compact Tab Bar.
On the Mac, the Show Color in Compact Tab Bar option can be toggled on in the Safari preferences under Advanced and then Accessibility.
Apple today updated macOS Monterey to change the tabs back to the original ta