If you have a wedding theme that includes specific colors, like autumn russets or spring pastels, then bridesmaid dress colors will be pretty easy to pick. If your color options are more open, you can go with a shade that will flatter your wedding gown or complement your flower choices. You can also choose colors that reflect the year's wedding trends.
Let the bridesmaid dresses do the heavy-lifting of tying together different elements of your theme. If you have three bridesmaids and want to choose three different complementary colors for them, pull it all together with your bridesmaid bouquets. By using the same flower varieties in three different colorways, one for each bouquet, you can integrate the look with a minimum of fuss. Roses and miniature calla lilies are great for this because they're available in so many colors. Add all three colors to your bridal bouquet, and you'll have created a nicely unified composition with a custom look and feel.
Let bridesmaids express their personal style by selecting a color and asking them to choose dresses in the same shade. When you lead with color, it can solve a number of potential problems while still pulling everything together effectively.
Your attendants need to get through the day in relative comfort, and the weather will have an impact on how fresh and friendly they'll be feeling for the duration. In spring and summer, opt for lightweight fabrics like chiffon and silk. In fall and winter, go with heavier satins or even velvets if you like drama. You can play with textures and fabrics a little depending on the venue.