Not sure, but that might be true more for ionic silver than actual colloidal silver. Grey and green colors in ionic EIS silver is indicative of larger particles, but at least when it comes to grey that is not true of true colloidal silver.
From the mesosilver website:
Once the bottle is opened the color of MesoSilver may change to gray depending on the atmospheric pollutants found in the air. This color change does not indicate a problem with the product and is not cause for concern. The effectiveness is not effected. The color change to gray is commonly found in large cities like Boston, New York, Chicago, Los Angeles, Detroit, Denver, and in many cities in California.