It will work for both.
In this cases I would select the white picker and click on the white dress. In many cases when I do this I tend to mess up other colors. Or atleast that is what I think is happening.
You could make 2 conversions, one for the dress & one for the rest. Then blend them with a layer mask. It takes less time to do it than it does to tell you about it.
This is blending exposures, but the proceedure is the same when it comes to using a layer mask to show/hide changes.
Make 2 exposures in RAW conversion.
1 is "normal".
2 is dark to hold the highlights. (Or you could make it a lighter conversion to hold the shadows.)
Open 1, then Shift-drag 2 on top of it. (Shift-drag keeps everything registered.)
Layer>Add Layer Mask / Hide All. (Layer Masks allow you to make non-destructive, reversable, changes)
As long as the mask has a double line around it (may have to click on it to get one) you can use a light colored brush to paint on the pic which removes the mask effect from the 2 layer. I might start with white @ 40% opacity.
Layer masks are non-destructive. If you make a mistake, just paint over it with a dark brush.
NOTE: all Layer> Adjustment Layers have a built in mask, & this works just great with them, too.
There's an example in post #6 here. Look for more info in the processing stickys.
...my question is this: I shoot concerts. Say I bring a white (or grey) card w/ me and put it up on stage and shoot it a few times (Wouldn't the lighting change the cards white balance use -adding color)?
Good luck with that! You're going to have to guess. Maybe start out with the custom Tungsten setting, & then post-process. Keep in mind that the correct WB isn't always the right WB for any shot. You get to make it look good to you.