Tonkinese eyes are open and yet quite exotic looking, neither round nor 'oriental' in shape. The GCCF show standard calls for them to be - "Large and expressive, more almond shaped than round and set well apart. The top line of the eye is slanted down towards the nose; the lower line of the eye is rounded." The eye shape is best assessed when the cat is relaxed, as it may have a more wide-eyed (rounded) expression when it is either excited or stressed.
Note: When looking at the eyes of a Tabby Tonk be sure to look at the eye shape and not the impression given by the pale 'spectacles' around the eye, which can make the lower line appear more slanted than it is. See pictures.
I came to know the Tonkinese after the passing of a beloved Siamese in 1990. Having been used to the vivid blue Siamese eyes I initially thought that aquamarine Tonkinese eye-colour was a little faded - but soon came to appreciate its incredibly subtle beauty. I found it interesting that, for many reasons, the eye-colour cannot be reliably bred for. The aqua colour in particular is so subtle that both ambient and reflected light can alter the appearance of the colour.
There are typical eye-colours associated with each of the three coat-patterns - shades of yellow/green with the solid coat-pattern, aqua with the mink coat-pattern and clear blue with the pointed coat-pattern. The fragility and variability of eye-colour causes some cats to have atypical eye-colour - for example cats with the mink coat may have more green tones than aqua or, in maturity, loose some of the green tones and show more blue. I've seen solid Tonks with lovely aqua eyes, but I've never a pointed Tonk with anything but blue eyes. The eye-colour is also affected by hormonal changes, particularly in females, with a tendency to shift toward the blue end of the spectrum, this behaviour is also seen in their Burmese cousins.
See also my slide show - Understanding Eye Colour