Zoom! Whitening can yield great results but is contingent on numerous factors, i.e. pre-treatment shade of teeth, age, density of enamel, etc. Your ulcers could have been totally prevented; they occurred due to inadequate isolation of the teeth and the bleaching agent is very caustic to soft tissue or gums and lips--they'll get burned unless they're out of the way! My practices always include take-home custom bleaching trays as part of the Zoom! treatment so patients can maintain what they paid for.
The level of whiteness right out of treatment had a lot to do with dehydration of the teeth; as they dry out, they get lighter. As they rehydrated over the next 24 hours, teeth will have some refractory darkening. The yellow tinge is the second layer of the tooth showing through, the dentin. It can be mustard-yellow in color and, as the enamel is thin and translucent at the tops of teeth, you get some show-through.
Zoom! is a great kickstart for whitening but nothing will get that deep, prolonged white look better than custom trays and regular, careful, at-home use.
Hope that helps!