Great photo! It really helps us understand what you are seeing. Since the day that you originally posted, I have been asking around Bullseye and have some more information for you now.
It very well could be color specific. In general, opalescent glasses are more liable to show food stains. We seldom receive reports about this, but we did get one long ago about pineapple staining a glass plate, and we were able to stain some opalescent glasses with lemon slices.
There is a product that we tried years ago to remove a food stain and it worked. It also seemed to prevent the glass from subsequent stains due to contact with food, even after the solution was cleaned off the glass. It is called "The Stain Remover That Really Works" by Novacan in Canada. It contains hydrofluboric acid, and must be used with rubber gloves, good ventilation, and caution. I am not sure where you can buy it, but I did get some results from a google search.
This is not a common problem. However, a way to prevent the possibility of a food stain would be to design projects so that they are capped with a sheet of clear or other transparent glass.
Let us know if you decide to try the stain remover, and if it works.
Mary Kay Nitchie
Bullseye Glass Co.
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