When I need to use an eyedropper to determine some color on the Mac, I usually take a quick screenshot and open it in Photoshop. But there are specific apps for that, like the newly launched Drop from developer Tapity.
Very friendly and simple to use, Drop works across macOS and supports several color formats including HEX, RGB, Swift UIColor, Swift NSColor, Objective-C UIColor and Objective-C NSColor (the latter, great for developers of apps).
When using the dropper, you can use the mouse wheel to zoom in and be able to select exactly the desired pixel; then, you can save colors to create a custom palette. It even supports the Touch Bar on the new MacBook Pro, offering quick access to your favorite colors.
If you liked Drop, enjoy it: it has a 50% discount on the Mac App Store, in promotion of launch.