Another preschool issue

I'm wondering how concerned I should be on this one. My 3-and-a-half-year-old attends a small preschool (like 10 kids). There are really no other children of color in our area, except our second child. The preschool teacher, who is extraordinarily kind and often picks my daughter up to help me with transportation because I'm legally blind and can't drive, recently questioned me about my daughter's ethnicity. It was awkward but not necessarily negative. The same day, my daughter came home from preschool and said she didn't want to go anymore. She has always loved going to preschool. This was quite extraordinary. I asked specific questions until it came out that kids at preschool had given her a hard time about being "brown." She said the teacher wasn't involved. I asked the teacher about it and she was bewildered and said she had not overheard anything like that. My daughter has gone back to preschool without complaint but without her previous enthusiasm. We talk about physical and cultural differences. We recently went to the big Romani music festival, where there were almost no other children but my daughter loved it anyway. A Romani woman comes to play with our children for half a day once a week and teaches them Romani language. And (just to head off the inevitable onslaught) we can't move. I would if I could and some day I may be able to but it will take some long-term efforts. Oh, the preschool teacher has agreed to let me volunteer to teach a series of multicultural exposure sessions at the preschool next year. Does anyone have any experience on how well that sort of thing works? And am I over-sensitive to be concerned at this point?