Open your picture and tap Effects. Select Corrections and tap Color Eye.

Then zoom in and put the cursor over the eye. Tap the settings button (the red circle on the bottom) and select to color and saturation you want.  Then tap Recolor. Repeat on the other eye and when you're done, tap Apply located at the top right. 

If you don't want to do that, or you want something cooler, then you can add these pictures.