Just found this:
first method works much better than the second IMO.
Even though the second "Complex" method is much better than the first, it only works on objects that are relatively mono-colour. I would suggest strongly a less crude way of doing it by using a "Channel Mixer" adjustment layer (instead of pasting over the original image). Then you can Paint onto the adjustment layer with white where you want the effect to be present.
In Channel Mixer you can for instance replace the Blue channel with the Green channel by choosing the Blue channel from the top pop-down menu - draging the Blue slider to 0 and dragging the Green slider to 100. This can then be done to the Red channel if it needs it by just choosing the Red from the top pop-down menu and dragging the Red slider to 0 and the Green to 100.
The beauty of adjustment layers is that they never permanently change the pixels - you can just turn that layer off and it is back to the original. Where the adjustment layer is painted with White = 100% strength of the effect and Black = 0% (with percentages of Grey inbetween)
Careful painting of masks will let you isolate the change to specific areas - then once you have spent the effort making the mask you can always use the same mask applied to a Levels or Curves Adjustment layer to re-colour that area to what it should be.
I hope this makes sense to beginners of Photoshop.