In times gone by the bride's bouquet was composed of white flowers to symbolise her purity. Although some brides still choose to have a white bouquet, many choose colours which complement the bride and bridesmaids' dresses as well as the general colour scheme of the wedding.
When choosing flowers it is worth bringing a sample of fabric from the bride's dress along with you. This can be a great help when matching colours. This applies even if the bride has a white wedding dress and wants a white bouquet because whites can vary in their shade and intensity.