How to get that "halo" effect

Sadly, the photo You posted does not work. Can You post a link?

 

So.my guess is a thin layer of tinted resin before the white resin is poured. So, in the mold you would pour the tinted resin before pouring the white resin in. Or it could be dye/ink/paint/(some form of coloring agent) that is painted onto the mold prior to the mold being filled with the white. Again, this is just my guess, I don't actually make eyes but I've watched tons of other resin videos and I am pulling my answers from the ways I have seen others tint resin in such a way.

 

If you are a fan of youtube, you can look up various resin casting (not just eyes). They do a lot of painting prior to pouring resin and it might help you get an ideal on how to execute it. I've seen people take pigments/inks and paint designs on items (trays, bowls, etc) before pouring their resin of choice. I suppose you could also paint the resin afterward and then coat it all in a clear coat as well before doing the eye...but that seems more complex in my mind.

 

In this video of Tianba making her eyes, which have that halo effect, it does look like the halo ring was done as part of the resin casting, like @errinreynolds was thinking...all of the bases Tianba has prepared at the beginning of the video before she starts the painting process already have them on.



I've tried shading a halo onto my resin eye bases with pastels before I start painting the iris, and that actually worked out pretty well for me-- you could give it a shot and see what you think. I'm not good enough at resin casting yet to try adding a gradient in my eye bases--- at my level of skill I'm overjoyed just to get an eye base that is solid white with no weird marbled swirls and doesn't have pinholes in the edges of the iris