1.Solicit your bridesmaids' opinions. Even though you will have to make the final decision (particularly if there are a lot of bridesmaids), it is a good idea to get at least a general idea from the bridesmaids. Are there colors they absolutely won't wear? It is better to have this information upfront than to suggest a deal-breaking color later on.
2.Consider the time of year and type of venue. The colors that make sense for a casual outdoor wedding in May will look ridiculous for a New Year's Eve, black-tie event. You should also consider the room in which you will be taking pictures, as well as your likely color scheme for the reception.
3.Find stores/lines/designers with dresses you like in the colors you are considering. You cannot simply tell your bridesmaids to buy a "blue" dress, as there are an infinite number of shades of blue. Similarly, two dress colors might have the exact same name (emerald blue, for example) but look different if they are made from a different material or by a different designer.
For example, the two following dresses, the first dress is in chiffon and the other is in taffeta, but they are the same color,red.
4.Try to see the color in person. Discerning color from a computer screen, if you are shopping online, is rather difficult.
5.Request material swatches. Even better than seeing the dress is having swatches of the material to take home (this is also important if you can't go to the store or see the designer's work in person). Having the swatches also allows you to show them to other friends, as well as to the bridesmaids, and to see how the colors look in different lights.
6.Make your decision. You might solicit the bridesmaids' opinions one more time at this point, or simply decide yourself given the information you have gathered.