The boutonniere should go with the rest of the group. For a prom or dance, whitw is always a good choice. Or if the tux has some color in it, go with a lighter shade of that color. For a wedding the boutonnieres should go with the color scheme of the wedding but a lighter shade.
The kind of boutonniere to get should be based on what you and your guest (if you have one) will be wearing. You do not want the colors to clash. Also you don't want a flower that smeels bad. Some simple choices that go with just about anything include white flowers like a rose, freesia or a small orchid.
The type of boutonniere you choose is a personal decision. There are things to consider when choosing a boutonniere such as type of flower, color, and time of year. Depending on the time of year, if a certain flower isn't in season, it may be more expensive to get. Also, you should think about what you'll be wearing.
If the boutonniere is for a wedding,the flower should be one that is a part of, or at least be the same color as the brides bouquet. If it is for a dance, such as a prom, then it should be coordinated with the color of the girls dress.