I don't have Miyo's expertise (look like she's unsubscribed).. but I've done lots of dyeing through the years..

It could be that your rayon was dyed while in solution.. meaning it was dyed before it was spun into the yarn that was woven into the fabric that makes up your dress. **usually*** that dye process is much more colourfast.

Keep in mind that anything you do to remove colour has the potential to weaken the fabric.. so if you really love your dress, you might want to think of another option.

"Your mileage may vary"