Once you have determined the formality of your wedding and have chosen the color or color range you want your bridal party in -- your parents are part of the bridal party -- you should talk to both your mom and the groom’s mother about their attire.
The formality of your wedding will dictate the formality of their dresses, specifically the length. A black-tie wedding requires a floor-length gown. Also, the formality of the wedding may dictate the type of fabric and look of a dress.
As the mother of the bride and host of the wedding, your mom gets first dibs on the dress re: color and style. She may want to chose a color and the groom’s color may need to avoid that color, unless you tell everyone to wear the same hue.
Don’t be afraid to take your mom and his mom shopping -- together or separately depending on geography, scheduling and the relationship.
Be clear as to what shouldn’t be worn. If the church requires arms to be covered make sure everyone knows.
If you’re not clear on what is and isn’t appropriate you risk not just bad wedding photos but an embarrassing moment or making someone close to you feel very uncomfortable.