Unhappy with veneers - very yellow in between. Why has this happened? (Photo)

Looks like the "yellow" is the underlying tooth structure. The tooth underneath has not been fully covered. Since you chose a very higher value (brighter) in the final restoration, the opaqueness contrasts against the base tooth shade. Breaking contact between the teeth, to wrap the restoration can hide the transition lines, which are evident in your case. Also appears like there is recession on your premolar areas.As with all information, consult your dentist before making any decisions regarding your dental health.Hope that helps!

It would appear from the photos that the veneers don't wrap around the tooth far enough to hide your natural tooth or that the gum tissue receded a little.. The dentist can redo them and extend them further back or behind your teeth. Hopefully, some of them are OK and you don't need to have them all redone. The space can also be taken care of by making them tighter together. It doesn't look like a bad job was done..they actually look quite nice so the dentist is capable of doing a good job for you.