For a long time I have been dissapointed with the eyes on my sitters during portraits and model portfolio shoots, especially for my studio shots using flash.
The reason being my studio can be quite dark and the sitter's pupils have to open wide to compensate and the proportional modelling lights do not have enough power to help.
This means that when I take the shot the iris colour is minimal.
I have learned from a book I read that if I have a light with a powerful narrow beam at the back of the studio pointing directly into the sitters eye this keeps the pupils small and maximes the iris and therefore eye colour and is not too uncomfortable.
This produces far more pleasing shots and my clients are actually commenting on the brightness and rich eye colours obtained.
The downside is that you may have two catchlights one that needs spotting out but if you put the beam next to the key light this can be avoided sometimes.