Ulmus parvifolia bonsai leaves are going yellow?

It sounds as though it is just taking on it's Autumn tints and soon it will loose it's leaves until next spring. They are not evergreen bonsia or no bonsia.

There are two varieties of Ulmus parvifolia.The larger leaved is hardy, but the smaller 'Yatsubusa' with leaves approx 1cm. long is evergreen , however even evergreens need to loose their older leaves. One cause is if it has had a move from warm to cool or vice versa, this can cause leaf fall. Is it well drained,? Sometimes overwatering can cause damage, as can excess ive fertilizer . Check to see if stem is green beneath the bark if it is then it will recover, or you could repot if no drainage holes in the container.
If it is the small leaved variety it needs frost protection
Good luck..

It will definitely prefer more food and water this time of year to less though, the majority of bonsai seem to fail through lack of water... make sure the drainage is good and water so the excess water runs through the roots and out of the bottom of the pot 3 times to ensure enough moisture - remember that the very nature of the pot design restricts retaining of water and nutrients so it is up to you to provide without over doing the feeding. They also like to be 'misted' which also helps reduce bugs on leaves etc.... some will go through cycles like 'normal' trees and will shed leaves, go 'off the boil' for a while and then come back nicely....water, food, mist and love!!