The Color Accent Studio Pro app for the Mac has been on sale in the Mac App Store at £7.99 but at the time of writing and for a short while it is a free download. This app just does one job - colour popping. On loading an image it is presented as a greyscale picture. Then it is a question of going over the area where you want the original colour to appear. Besides that you can select your own colours to add to the picture. The result is saved as a png file. I haven't found a list of which import file formats it can handle. My test photo was a jpg.
One quick example which must have taken me all of 20 seconds to produce:
This app is at v1.0 and for me there needs to be at least three additions to make it worth anything like the original asking price:
1) ability to crop
2) adjustment for brightness & contrast
2) choice of saved formats
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