Don't worry about wearing the same color as the bridesmaid's or mothers. You have no way of coordinating with everyone in the wedding party, however do try and avoid white.
Do wear something feminine and appropriate, out of respect for the host.
Don't wear anything too sexy or reveling. This is the brides day, it is her time to shine, don't take that away form here. There are plenty of ways to still look great and not be the center of attention.
Even if it is a casual event, it is always better to be over dress than under dress. Stick to pastels and pretty sun dresses for daytime affairs. Little black dress for night or after 6:00 pm.
Informal Daytime: Short dress, dress shorts and cute blouse.
Informal Evening: cocktail dress, little black dress, or something bright and elegant.
Semi formal daytime or Evening: short dress or cocktail dress. A long dress made with lightweight material can be appropriate as well. You can even do a nice pair of slacks and blouse, keep it simple but dress it up with some bold accessories to make a statement.
Formal evening: Long dress or dressy short cocktail dress with fabulous heals and glam accessories
Black tie: Long gown, extra glitz diamonds and fur please!!!