I think it is high maintenance. I have been both a bride in a non white dress, and a pregnant guest at a wedding. When I was the bride I didn't tell anyone that I wasn't wearing white, and certainly never asked my guests what they were wearing. I didn't care. If one of my friends had rocked up in the same dress as me (it wasn't a wedding dress) I would have made sure we got lots of photos together because it would have made for a great story (although, I think this only works when the bride isn't wearing a traditional gown!)
When I was a pregnant guest I wore tights and a long dressy top that cost me $12 from valley girl with some ballet flats from K-Mart. Did I look dressed for a wedding? Not particularly. In fact, the bride told me I should have just worn something I already had in the cupboard and not spent the $12...they got married on my due date and absolutely did not expect me to spend money on clothes for their wedding, let alone ask me to buy two outfits!