In the below image, the bottle neck 's color is green. How do I change the color of this to make it white using Photoshop?
Get the image into Photoshop, get an Adjustmentlayer over it, adjust it that the green part will be white:
Get only the part which was green to be visible for the adjustment layer:
To get there, use the Quick Selection Tool to select the green part of the Bottle.
Then inverse the selection to get everything else but the green part.
Now (if not selected, select the Adjustmentlayer) and hit alt+backspace (on a mac) and the Adjustment Layer only applies to the white area, not the black one.
Doubleklick on the Adjustment Layer to get to its settings. Set the controllers like in the image. Now it could look like that, with a little bit more tweaking its even better ;)