These threads always seem so obtuse. It isn't so much that the person in the cream/offwhite/ivory dress is going to be "mistaken for the bride".
It's about giving the bride the one day in her life when she is the centre of attention, where she gets to wear the style of dress that makes her the Bride and where other people give her the space to be the star of her show.
Anyone who has to ask "is this too bride-like" knows they are pushing it close to the edge, it's a hair's whisker away from being the type of dress that could be suitable for a bride, so why bother asking. Just be the good person and consider something else