I had a request for a closer view of the eye on the Roman figure. I also grabbed an image of the Dwarf's eye since they're about the same size and the painting approach was the same. To recap, in each I started with the dark shadows around the eye. Easier to get this into all the cracks and crevices without worrying about screwing up your carefully painted eye, so that's why I do this first. From there I paint the eye a reddish pink. This is covered up by the white of the eye but will still show through at the corners. For the white I use off whites. If you use pure white it will be too bright and make the eyes look cartoony. With Reaper I use Weathered Stone and then mix in some Leather White to get a little bit of shading. They eyes are so small even on a 90mm scale figure there's no need to go overboard with the shading, but a little bit is nice. Don't go all the way to the corners, leave a little of the pink showing.
To do the iris I start with a dark color, either dark brown or dark blue. Begin with a little dot and then slowly work it into the full size. You can also start with a vertical line if that's easier. If you're doing two eyes (which you often are), I do this with both eyes at the same time. That why, as I increase the size of the iris, I can make small adjustments to keep them looking in the same direction. Maybe expand one eye to the left a little more and the other one to the right. Just see what needs to be done. Of course if the iris is too big or misshaped you can correct with your white mix. With the shape and size correct I go in with a lighter version of the iris color. I try to paint out from the center leaving a dark border to the iris. I'll then go in with a lighter shade or even two to give some variation.
The pupil is pretty straightforward. Just a dot of pure black. I use a 000 size brush though you could probably also dab the tip of a toothpick in the black and add the pupil that way. If you're having trouble you might find it easier coming in from the top of the eye.
The last step is to add the catch light. This is optional and something I wouldn't bother with on a 54mm or 28mm size figure. But for the 90mm scale it's something I like to do. Now I will use pure white for this part. Just like the pupil, add a small dot off center. For the Dwarf I actually did a double dot, one big and one small. An alternative to adding a catch light is to coat the eye with a gloss varnish. I think this trick works better on larger scale figures like 150mm and 200mm scale busts, but you can try it on the smaller scales and see what you think.
And here are the eyes magnified by a factor of 2. It's a little easier to see the variation in the iris color and the double catch light on the dwarf.
Oh, and don't worry about going outside the lines and getting paint on the upper or lower eyelids. This is very easy to correct with your skin shades. When painting around the eye I use a dark line along the top for the bottom edge of the upper eyelid (and also the eyelashes). Along the bottom of the eye, the top of the lower eyelid, use your skin highlight color.
Keep in mind all the detail I'm doing on the eyes is because this is a 90mm scale figure. Attempting this on a 54mm or 28mm is beyond my abilities, so I do a simplified version but the idea is the same (off white, dot for iris/pupil or just merge into a single dark dot, upper and lower eyelids, etc).