You’ve probably seen photos where most of it is black & white with one bit colorized. If you have a photo with a strong contrast between subject and background, it’s pretty easy to do in Photoshop.
Earlier today I took this picture of roses blooming in my neighbor’s garden. I used the largest aperture for my lens (F5.6) to bring out the subject and blur the background slightly.
Since there was a pretty strong contrast between the flowers and the background, it was pretty easy to use Photoshop’s color range selection tool to select the flowers only. I then inverted the selection and made an adjustment layer based on the selection. In the adjustment layer, I used hue/saturation to desaturate it and darken it slightly.
Here’s the final result.