When the vast majority of my friends were getting married (also my sister) I enjoyed helping them pick out a dress to wear for their rehearsal dinner. Most girls will wear white to the dinner but there is no written rule that one can’t wear color or pattern. If I were getting married I am not sure which route I would take, probably would end up wearing white as I personally love little white dresses, but I am not oppose to a little color. Wearing a cocktail dress that does not look like it was made for a bride means you can rewear the dress again.