One of possibly several explanations is that these color changes are occurring at areas which have been formerly repaired. "Re-glazing" over a repair involves using some sort of material which takes on the appearance of glaze, but is not the same as the original silica-lead glaze. Many repairs look "perfect" initially, but over the years will age and discolor. I don't know the size of your discoloration areas, but some restorers extend the "re-glazing" well beyond the restoration, and I have even seen entire pieces "re-glazed". Although this discoloration may occur under any light conditions, direct sunlight would be the worst location to display your pieces.
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