In this easy tutorial I will show you how to take a color photo, change it to black and white, then use a layer mask to bring out some colors.
First open this picture
Duplicate the layer so you have 2 copies of the image.
Then on your menu bar go to image>>adjustments>>desaturate or hit the shortcut key combination shift+ctrl+u on the top image.
Now your picture should be black and white
Now we want to add a layer mask by either clicking on the layer mask icon at the bottom of your layers palette or go to Layer>>layer mask>>reveal all.
Once you do that you will see the layer mask beside your picture
Now grab a soft paint brush, I chose 9px as my size. Make your background and foreground colors are set to black and white default. You black color will erase parts of the image to reveal the color and if you change the color to white you can paint the image back in case you mess up.
So zoom in and uncover the colored parts you want
Here is my result after coloring the flower
You can do any part you want, her eyes, hair, nails, shirt, etc.. Here is a variation of some of those: