I use a good, opaque, thin, Flat White paint to base coat the eye. Poly Scale has a good acrylic shade. In many cases, I paint the eyes first and then paint the face around them. Since the eyes are the hardest part of the face to paint, if you do them first, and screw them up, you just have to strip the paint off the two eyes and start over. If you do the whole face and its come out super... then you screw up on the eyes... well you lose all of that effort!
I will tint the white with a wash of brown, black, grey, etc. This outlines the eye and also tones down the white. You can use fun colors like yellow, red, pink for monster effects.
This is a big resin Power Girl figure. Her eyes were done in white and kept "bright". I think I put a dark wash on the plain, unpainted eye first, then filled in the white.
The Aurora/Moebius Monster Scenes "Victim" was painted similarly and with the same paints.
Dr. Deadly's eyes are a bit darker...
Nosferatu's yellow eyes were done with a thin wash of yellow enamel paint over the white base.
I used red on the Polar Lights Witch figure. She was painted with Games Workshop/Citadel paints and ink washes.
Dr. Jekyll's eyes were well sculpted and easy to paint. FYI I paint teeth the same was as the eyes... white base and tint with washes and/or a light dry brush.
I am sorry if some of the pics are a bit blurry... most of my figures are up on a shelf behind my counter at my shop... so I used my camera's Macro lens to take the shots.