First you should choose the colors of your wedding (the colors of the linens, bridesmaids dresses, favors). Then, research flowers that compliment that color scheme. Also, try and consider your budget and research the flowers that are in bloom at the time of your wedding.
It depends on the season of the year, and the theme you are using. Their colour may depend also of the colours of your cake topper - it happened with me.
You should use the flowers that you like. You can even have more than one kind in a bouquet. I chose daisies because I love anything that has daisies in or on it. I also used yellow roses, because they went so well with daisies. What color are your dresses? Mine were green, blue and yellow 2 colors in daisies.
Choose flowers that are in season at the time of your wedding to keep costs down; they should match your color scheme and fit in with your overall design.
Usually, people choose flowers with the color theme of their wedding in mind. For example, if your wedding colors are pink and purpole, you would choose pink and purple flowers,