This video tutorial by VvCompHelpvV gives you two methods to achieve the Sin City effect in your photos using Photoshop. The Sin City effect is making the main subject of the photo appear colored while the rest of the photo looks black and white. In the first method the subject should be selected using a pen-tool using the settings mentioned. After that right click on the selected areas and make the feather radius 3, inverse the selection and go to image > adjustments and de-saturate and your done. The next method uses an adjuster layer. For this add a new adjustment layer - gradient map and make the gradient black and white. Select the brush and invert the black and white color and paint in the subject as shown.
Have an iPhone? Check out all 200+ new features coming in iOS 13.