Medium

Glazes allow the colors underneath them to still show through, and you can see this if you look at the doll's eye above. You can also do wet-into-wet techniques with this medium, which will allow you to blend one color into another and create gradient effects.

Painting realistic eyes: After painting the whites of the eyes with titanium white paint and sealing with gloss sealant, you can start painting the iris in layers with glaze. Mix some medium with one of the darker-colored paints. If you look at your own eyes, you'll see that your iris does not have a sharp delineation into the white of your eye- the color blends a little bit. So, by painting the iris in 3-4 glazes of thin color, you not only attain a more natural-looking repaint, but it's also more forgiving in case you aren't able to paint a perfect circle on your first try. With a really thin glaze of paint, you can refine your circle by applying more layers of glaze, eventually attaining a dark-colored iris, which will be the foundation for the rest of the eye's paint. After you've painted the dark iris, you'll apply light-colored paint to the iris in layers of glaze as well. After that, you'll paint glazes of the darker flecks of color on top of the light-colored paint, and that will give the eyes depth and realism. Mix a very small amount of black paint with medium, and I used this mixture as a glaze to create a shadow on the eyeball underneath the upper eyelashes to create more depth and realism. You really do need medium in order to paint a doll realistically!