The first explanation that comes to mind is that there is some amount of iodine in your husband's saliva - either from the gum cleansing, or from the toothpaste, or from seafood (if he eats these at dinner). Iodine in saliva will be colorless, but if it comes in contact with starch in the pillowcase, it will turn purple.
A cotton pillowcase may have starch added to it. Or, since cotton itself contains some amount of carbohydrates (starch is also a carbohydrate), it could react to the iodone, if any.
Another explanation ( a bit far-fetched), is the excretion of Niacin in the saliva. Niacin turns purple when it comes in contact with potassium or with chemicals like cyanogen or barbituric acid. But then why should these chemicals be on your pillowcase? I have no idea - like I said earlier, the idea is a bit far-fetched.
I think that the easiest explanation is the reaction of iodine and starch. Perhaps you could try a silk pillowcase - if there is no purple color, then this could be the likely explanation.
I wish I could have helped you more.
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