I don't have an answer for you, but since you probably don't want to use this day to make a personal statement (if you do, wear the red!), I would just as the minister/priest/pastor of your church what is normal--and then chat with other parents to see whether their children are wearing all white or not. I guess I feel like there's a time to not worry about what others think, but sometimes worrying about what others think is a way of respecting tradition and other people's sensibilities, and this would be one of those times.
I'm assuming you're Catholic...I'm Lutheran and have never seen Lutheran children wearing white dresses (I didn't, 20 years ago, even) for First Communion. When you say you've seen them wearing white, I assume you mean in your particular church? If you're just referring to seeing it ingeneral, though, I'd definitely ask your church, since some churches do not do white for girls at First Communion--it's more casual than that.