I need to verify what exact color an app is using on a specific element.
I want color values in RGB and/or hex.
I can make a screenshot and send it to a PC.
Best would be an iOS or macOS app, but online or Windows would work as well.
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Sign up to join this communitymacOS has a built-in application called Digital Colour Meter. It's installed by default, in the /Applications/Utilties folder:
Here is me using it to find out the background color for Software Recommendations' menu bar, which is RGB(23, 77, 96):
As @Tetsujin notes in the comments, via the menu View → Display Values you can change the output to hexadecimal format:
(of course, I could've found these color values with the browser's developer tools, too, but you get the point...)
There is also an app from the App Store named Color Picker. Click on the magnifying glass and select a pixel that has the color you want. You get the color in RGB and hex. The app screen is shown below.
And clicking on the magnifying glass to get the colors used in the book cover yield the following image. The numbers in the magnifying circle are the rgb values for the pixel the red square is on.