As is true of many procedures in Photoshop, there are several ways of doing selective color. This is a quick and easy method.
First open your photo in the photo editor of course.
Then go to Layer – Duplicate Layer
Then go to Image – Adjustments – Hue / Saturation
Drag the Saturation slider all the way to the left to make the photo black and white.
Then get the ERASER tool from the tool box on the left side of the screen. It is usually the next tool under the clone stamp tool. Just hover your mouse over the tool, and it will say Eraser.
When you have selected the Eraser, go up to the Brush options box at the top of the screen, and click on the downfacing arrow. Set the Hardness of the brush to 100% so you will have accurate control of selection. You may also need to raise or lower the brush size to suit.
Erase away any black and white from where you want the color to be revealed.
When done go to
Layer – Flatten Image
Then just do a Save As to wherever you want to save the photo.
If you have a color picture, open it, copy it and paste it onto a new layer. Make it b&w (go to Image –> Mode–> Grayscale, or Image –> Adjustments –> Desaturate). Then select the element you want to appear in color with the Magic Wand or the Magic Eraser tool in the b&w picture. Delete it. You should be able to see it now in its original colors, while the rest of the picture is b&w. (Actually, what you see is the object in the original underlaying picture, since you deleted it in the top layer). If the contour is fuzzy, retouch it with the eraser tool on the b&w layer.
If you don’t want the object to appear in its original colors but just one color, it’s the same process, only you play around with hue and saturation (Image –> Adjustments –> Hue/Saturation) until you get the color you want before copying and pasting the picture onto a new layer.