Convert your picture to black and white. better way to do this is using Black and White adjustment layer on photoshop because you will have much more control of image contrast.
Once you have your Black & White window open, you will find you have a lot of control over how that black & white image will look. The most important thing to consider is the contrast. Using the preset at the top of the window you can predefine the settings according to the most common black & white filters. If you want to adjust a specific color, you can use the color sliders below. I usually use the Cyans slider to darken the sky a bit.
Every layers adjustment comes with it´s very own Photoshop layer masks. We´re going to be using that layer masks to bring back some color in selected areas we wish to emphasize. When you click on your layers adjustment you´ll notice your Background/Foreground color will automatically be set to black and white. Because your mask is white, we´ll be painting with black.
If you accidently paint outside of the subject simply press x on your keyboard. This will switch background color with foreground color. Now that you have the white selected, use it to erase the excess.
Finally, if at any moment you want to check out how your layer masks is going in full view just click on it while holding down ALT. This way you can check if you missed a spot.