In the Photos.app I am using Markup Toolbar in conjunction with screenshot editor.
How can I fill an area I selected using Instant Alpha with a specific color?
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To fill color using instant alpha:
The Toolbar on the Preview App is handy for basic editing, but more detailed work would require a proper photo editing application.
The Photos app provides some basic editing such as color correction and rotation. However it does not (correct me if I am wrong) allow for working Alpha channels or layers.
There are several third party apps available on the Mac App Store that allow for fine photo editing. Pixelmator comes to mind.
If you have Pages or Keynote installed and you just want to make it quickly you could do this. However, this is just a quick and dirty way to get the job done.