Shuttered Life

First, open the desired image.

After opening the image, convert it to monochrome. You can do this by selecting the channel mixer or the black and white option for the image. In this tutorial, I prefer to use the channel mixer then click on the monochrome checkbox then click ok. You can play the RGB channels of your choice.

Then focus on the toolbar on the left. Make sure the color picker is set to BLACK and the BRUSH TOOL is currently selected. Then paint on the image you wish to be colored again.

Here are the results:

Before:

After:

Comments will be highly appreciated.