I am trying to understand what is the effect that is applied in the dashboard icons of Facebook and LinkedIn android application. They change the icon when the user press it, replacing to another darker icon. In the following links you can see the original icons.
I am not a photoshop expert so I can't figure out what is the effect that was created in the hover icon. It seems to be a gray transparent mask over the original image. I am thankful for any answer to this question that can lead me to the same effect.
Also, what is the best approach to have the same border lines in my icons. They have smooth borders using a lighter color.