Am going to use below picture,
In this image, am going to select just the red roses in the bouquet and in the dress so I can leave them in color while converting the rest of the image to black and white
1. Open your picture in the photoshop
2.Select Eyedropper Tool then follow below proceedures to open Color Range dialog box
3. Click once inside one of the roses to make my initial color selection and you can see your initial selection in the preview window:
4.To add more areas to your selection, first makesure eye dropper tool is selected then press and hold Shift key and click on more shades of red in the roses
5.After adding more areas to your selection,Then fine-tune theselection with my Fuzziness slider, set Fuzziness to 36,if you are happy with the selection then check Invert Box, then click ok to complete selection
6.Add New Adjustment Layer , makesure background layer is selected,Then choose a Black & White or if its not there choose Hue and saturation from the list that appears:
7.Set Saturation to -100 and if you are happy with the result then click OK
Here is the final result
