1. Open your target photo. On the layer palette, it is the Background layer. It's up to you if you want to duplicate it. 




2. Create a new layer and name it as "eyes". Zoom your image as needed using . Just press ALT+click to switch between the zoom in and the zoom out. Set the foreground to any blue color like #3a5ef8. Click the Elliptical Marquee tool and drag a selection just fit for the eyeballs then add a selection to the right eye but make sure you click the Add to selection option. When you already position the selections, press ALT+Backspace to fill them. Press CTRL+D to deselect.




3. Click the Add Layer Mask on the bottom of the Layer palette. Change the Blending Mode to COLOR. This becomes a little bit transparent. Choose the Eraser tool and with the thumbnail layer mask selected, start erasing the part that comes out out eyes.




4. To finish, lower its opacity to about 30-40% depending on you.





So start experimenting your eyes color. Green eyes and brown eyes are good, too.