It's easier than you'd think. Today we're going to start with a Hujoo eye. The default ones many of them come with.
First you need to take out the original detail.
Protect the front lens with putty and drop a little nail varnish remover onto the sticker. Let it soak a little then begin peeling it off with some flat ended tweezers. Don't worry if the inside of the eye gets scratched. It gives the eye more depth. Add more remover as needed.
Now you have your blank. Choose a colour of nail varnish you really like, I find glitter works great here, and begin adding. If you want a pupil paint that first and let it dry before adding the iris colour. It's as easy as that and very effective.
Lol my messy fingers. This is a full glitter eye that shines very brightly. And they're not just for Hujoo dolls. Here is that eye presented in my SD Helen's head.
Here is an eye with a pupil...
I hope this was helpful and if anyone wants any more advice feel free to ask.