Just experienced this with a different color of glass tile. What I thought was slate blue and almost white ended up dark teal and pale sea foam green. Changed the direction of the room completely. A sample board would have prevented the surprise completely.

My contractor is not going to be pleased with me requesting a sample board. But I might. Does that involve the tiler getting a few loose tiles and putting it on a piece of sheetrock to simulate the finished product, using the same materials they will use in the job? Is there often a cost for this? I'm seeing the color variation in the glass just from the samples of the same collection I take home that must be from different lots. So I'm really scared. I assume a sample board will also have to be made using the same lot as the order that would be placed......

also, specify white, not gray, thinset.

A sample board

It's just smart.

Also be sure to look at your tiles on a vertical surface vs. laid out on the table/counter - my pale gray/green/blue glass tile looked darker and more green so I was able to get my mind right. Google images of your possible tile to see installed photos.