How do I politely ask bridesmaid to color her hair?

I'm getting married this fall. I found out one of my bridesmaids has decided to let her hair go gray naturally, which would be fine- but by the time we get married her hair will be half gray. It's already grown out quite a bit. I was pretty shocked when she said this, and we are so close to the wedding. I didn't say anything at the time. I really wish she had waited to do this until after the wedding. How am I supposed to ask someone to dye THEIR hair? I feel really uncomfortable about it, but at the same time this is obviously a really important day and I want everyone to look nice. How do I approach this without offending her or looking like a bridezilla/control freak?