This is the fastest method I know for changing the color of something in a picture using GIMPPortable, or Photoshop. You make a hasty selection with the lasso tool, feather it, and then adjust the hue, using the option to adjust only the blues in the picture. This prevents unwanted color changes simply.
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In my picture, I want the blue car to be green, but not the carpet, or anything in the background, so I need to roughly select the car.
Click on the lasso tool. Drag a quick selection around the car. Tweak the selection a bit using to add to the selection and to subtract from the selection. You do not have to be perfect with this. Not even close.