How to highlight one part of a black and white picture?

Is there an app for the iPhone to do this or do I have to do it on the computer?

If you mean "restore colour" to a part of a photo, that would be an unequivocal no. There's no app on a phone or computer that will restore colour to a black and white photo.

If you mean "highlight", as in draw attention to a particular area; then I can be of great assistance.
The app you want to use is "snapseed." There are similar apps outthere that can do the same thing, but i found it to be the most user friendly, and cheap. Alternatives include, "photoshop mobile" and "iphoto"
- "selective adjust" will allow you to make a certain part of your image brighter / darker. I.e. Make the non-important areas darker, while highlighting the one part to be brighter.
- "Vintage" - In the vintage selection, you can add a vignette by adjusting the "centre size"
- "Centre focus" will allow you to make the one part sharp, while the app softens / blurs the rest of the image
- "tilt shift" will allow you do something similar to "centre focus" the difference is that "tilt-shift" will allow you to work on a plane; while "centre focus" works around a pre-determined round shape

I think Sagara went into some space time continuum… Never to make any sense again.

Highlights and Shadows in Lightroom or Photoshop can bring out the details in your image.

I doubt very strongly that Apple has any iPhone apps that do it.
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