The look of stained glass changes with different lighting and viewing angles. See: Vine Maple Door, Crooked Fir #2 - Moon, Iridescent Dragonfly and Iridescent Damselfly. Especially note the extreme example of the Iridescent Damselfly where the perceived colours changes drastically with a change in lighting.
There are strong limitations to how well computers can display glass colours. The colour I see on my computer monitor or printer is not necessarily the same as what you see on yours. For a great description of colour on computers, see the Color Matters web pages. The same site also has a lot of other information about color.
Even when viewing samples of the actual glass, there may be differences. As noted on the Spectrum Glass web site:
We make every effort to grade each sample so that it is reasonably representative of a product’s color, mix characteristics and light transmission. However, in some glass types, it is impossible to fairly represent the appearance of a large piece in a small sample.
In the final analysis no matter what is used to approve the glass ahead of time it will be imperfect. In the end you will need to trust that I as an experienced stained glass artist will know how a piece will look when finished. I promise I will create the best looking stained glass possible for you.
Thank you, Timothy Atwood