Personally I'd go the black rather than white. We had two people show up to our wedding in white dresses (both extended family members - one from each side). It didn't bother me so much on the day, but looking back at photos it does look a bit funny IYKWIM? Ask your hubby what he thinks since you don't really know the bride that well, she might be a total bridezilla and freak out at someone else wearing white

Though if you wear a different coloured skirt and maybe a jacket or a shawl or something that makes the white stand out less that might work.