Sizing? Starch can look grayish if it piles up, so maybe the sizing in the fabric can as well.
Once I went to KQ, and a woman, well-intentioned but reeking of the McD's up the road, grabbed the fabric squares I was using to match and started commenting on them. I retrieved them Very Quickly. Once at home, I ran my iron over the samples out of curiosity, and sure enough, there was grease on them.
Hugs,
Charlotte