I have just tested the highlighting and background colors on the notes. I have missed the possibility to highlight text, so I am very satisfied. And even with different colors. Great!
The only issue is that I cannot get the transparency function work. I only get 100% opacity when I choose a custom color. The preset colors are working well, both in dark and light mode because they are transparent.
The preset note background colors are good in light mode, but hardly showing in dark mode.
I have also set a custom color to a project category, and it turned out very well. I am very fond of colors.
I think it is worth paying for Agenda not only because of the premium features. I see it as I support all the development. I am using Agenda as my main note taking app. It helps me think organized. And the highlighting makes it even better to quote text and work with it.
I am using iPad Pro. Have not tested on iPhone yet.
Not yet at this moment I’m afraid, with the exception that you can highlight using the default color if you use the markdown format ==text in between the double equal signs gets highlighted==, which should also work if you use this in the x-callback-urls and shortcuts.