If you can accurately make a selection around the dress, you can easily change the color of it, especially to white or black. Here's how I would do it.
1. Select the dress.
2. Open the Channels palette and pick the channel that has the lightest colored dress. Since the dress is reddish, that would be the Red channel.
3. Click on the Red channel and opy the selection from the Red channel. Edit > Copy
4. Now select the Blue channel and making sure the selection is still on, do a paste. Edit > Paste
5. Repeat for the Green channel.
6. Select the RGB channel so that the colors are back to normal. The dress should now be gray.
7. With the dress still selected, use the Levels or the Curves adjustment tool to lighten the dress to white. If using the Levels tool, play with the mid-tone slider and the white point slider to get a good balance of white and light grays.
Here's a quick and dirty example:
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