This is a very easy effect,
I'm new this is my first post, I'm sorry if this is too simple of a tutorial. I'm sorry if this has already been posted
1. Start By Choosing an image
I did a Google search and found this flower and since i like flowers and they are easy to test on i choose it.
Sorry of the large size.
2. Choose the Magic wand tool
3. I usually turn the tolerance down a bit, 50% Will select too much on most images (I used 27% for the flower)
4. Select all of the area of the image that you want to stay color
5. Copy that area and paste it into a new layer called color layer
6. Deselect the selection with Ctrl+D
7. now choose the background layer
8. Then Go to Adjustments and click black and white
9. You will now notice only the color layer has color(obvious)
10. Now merge the two layers.
11. I usually use the curves tool and edit the luminosity after merging the images to make the image seem more realistic. I usually also add glow to add vibrance to the image.
12. You should be done and should have a result like mine
Hope you all like this tutorial.