As a rule, the content of a session has been discussed in advance, but it can happen where the hypnotherapist is using imagery you aren't seeing or don't like.
In most cases, if the hypnotherapist keeps saying "green" and you are seeing "blue", you will simply see blue and go on. For most things, the exactness of the image is not the point. If you find it jarring or the hypnotherapist is using an image you can't deal with, you will simply open your eyes and correct what is being said.
I had one client who didn't reveal a phobia to me. With her in a trance, I was using a flock of birds moving in formation in the sky as an image. She opened her eyes and asked if could skip the birds because birds are "too pecky". I changed images and she was back under, more deeply than before.