Use the Magic Wand tool to select the dress colours (note you don't have to make the selection complete at this stage);
 Go to the menu bar at the top, and select:
Layer -> New Adjustment Layer -> Selective Colour
 Drop down the different colours to change, and adjust the sliders. Note that selecting "neutrals" will institute a global colour change, so for more subtle effects, you might wish to change the actual dress colour itself first. For example, if the original dress is green, drop down the combo box, select "greens", and then manipulate the sliders. Choose between relative and absolute modes as required. If you need a major change, then "neutrals" is your first point of call, as that will make wholesale changes to the colour.
 Once you're satisfied, click "OK".
 Any gaps in the colour of the dress can now simply be filled, by selecting an appropriate brush size for the brush tool, and painting white onto the mask that's just been created in your adjustment layer. As you do this, you'll see the underlying colour changing. For more subtle effects, change the white to a neutral grey before painting.
 When you're happy with the result, you can merge layers later if you wish, or you can keep the adjustment layer separate, and leave the original unchanged.
 This also works with hue/saturation, but needs more care and attention if you're going to use this mode.