Let's start here: "a groomsmen is doing black roots and bright blue." So that's ok, but hers isn't?

And I've heard of bridezillas, but one that dictated the exact hairstyle for their bridesmaids is a new one for me. So if they had shorter hair and their hairstyle wouldn't work for them, would you make them get extensions?

And you do realize that you're asking your "best friend" to basically not be herself, but something that you want her to be, simply for your enjoyment, right? As someone who is married and had an amazingly fun time and didn't have stipulations in our wedding, it's not worth trying to control everything, it's just not. Try to actually enjoy yourself. YTA.