Your bridesmaid's hair is purple. Your future sister-in-law has multiple tattoos. Your brother has extensive piercings. Do you mention that purple might clash with the red bridesmaid dresses or ask your future sister-in-law to cover up her body art so no one is offended or your brother to go ring free for a day?
People are different. Their character and style don't always match up to each other. So what do you do if you're not comfortable with the appearance or other aspect of someone who will be in the wedding party? No easy answers here. It's hard enough changing yourself, so trying to change someone else is near impossible. Nor should you try and change them. Whether it's a best friend, family member, or future in-law, you need to remember that all people are unique. There are going to be things you like and others you don't and just because you are getting married doesn't mean you have the right to infringe on someone else's style.
If you're super close to someone and feel you can request them to "change" their appearance, give it a shot. But you might risk losing a friend or creating an uncomfortable dilemma. And if it happens to be a future in-law you aren't chummy with, you're bound to strain your relationship even more with such a request. It's best to let nature take its course and allow people to make their own decisions. In some cases, they might surprise you by doing exactly what you hoped for. If they don't, chalk it up as out of your control and let it go.
Your wedding day is a special and memorable day for lots of people, especially you and your partner. Choosing to focus on someone else detracts from the purpose of the day. Since you have no control over others or what your guests might think, decide now not to let it bother you. Identify the quality you admire in each person in your wedding party and then tell yourself this day is about love. And that goes beyond appearances.