. Move the flash away from the lens and/or use "red-eye" adjustment in a graphics editor
Would red-eye adjustment work on pet-eye? Pet-eye is often silver (especially with cats), so would the specific red-eye tool work?
(Photoshop type stuff is still your best option, though)
Or any of the other image editors. Or, print the image and use the appropriate markers to color in the pupil, darkening it -- which is what we did before digital imaging when someone was foolish enough to shoot people with flash-on-camera.
. I'm not sure. I've never done more than watch someone else do red-eye removal. . I do know that moving the flash will work - at least with humans. Been using that "trick" for over 35 years.
That will prevent it, certainly, but the author wants to correct it in existing photos..
The concept is still the same, though the color may differ: Use a darkening filter on that part of the image. You may have to do it by hand unless your image processing tool has a "flood filter".
Use one of the photo editors and recolor the eye to the correct color. It's easy since it's a small part of the photo and is one color. For realistic looking replacements add a "catchlight". Look at good photos of people and animals. There is usually a reflection of some light in the eye. In some professional photos if you look close you will see a reflection of the umprela reflector that the photographer used. That adds interest and makes the eye look brighter. Animals are harder to photograph with out camera eye because they have better night vision and that's because the inside of their eyeball is more reflective than ours and their Irises are larger than ours.