The yellowing she is talking about is found on the white part of the eye. It is due to the liver function but the horses gums would also have a yellowish tint to them also. What she is talking about is similar to jaundice in people.

What you are talking about does sound like allergies. I would have your vet our to check the eye(s) and make sure that he hasn't scratched the eye in the process of itching, which can cause an ulcer. If their is no scratches in the eyes then your vet can give you a eye ointment with an antibiotic/steriod in it. You have to make sure there is no scratch in the eye before using this or it will make it worse very quickly. Also it is best to keep a fly mask on at all times. My horses wear there fly mask 24/7 during the summer. Just make sure that you look under it and check their eyes daily.

Call your vet and have him look at the eyes,

Good luck and let us know.