The latest update of Lightroom, version 4.1 includes an additional de-fringe tool and it really makes light work of green/purple lens fringe artefacts. Here is a working tutorial on using this feature, as well as some other elements to consider.
I managed to stumble on an image which demonstrates this feature perfectly, and demonstrates the amount of control that you have with the de-fringe tool.
Looking at this image from a regular viewing size doesn’t show any issues
However on closer inspection around the trees in the background, there is considerable purple fringing. This is caused by the high contrast between the green of the trees and the back lit nature of this scene.
I am easily able to remove the purple fringing by selecting the de-fringe eye dropper tool, and tell Lightroom exactly what to correct by hovering over the affected areas.
Notice that the “show loupe” is turned on and the scale has been modified to give a wider view of the actual pixel view area. The eye dropper tool automatically selects the purple colour once it finds the fringing. Clicking on the affected area allows Lightroom to automatically adjust the purple fringe amount slider, along with the purple hue slider by the desired amount.
the effect can be clearly seen,
however, on closer inspection around the scene, this modification has had an impact on another element in the photograph that was the same colour purple as the fringing. There is a purple house in the lower half of the scene, which now has some of the purple tones missing.
(as compared to the previous setting before the de-fringe tool was applied)
fixing this is quite simple, the amount of de-fringe can be manually adjusted using the purple amount and purple fringe sliders (green is the same). If the purple amount is reducing from 20 to 9, the house regains it’s colours, and the trees regain a tiny amount of additional purple fringe, however, if we adjust the purple hue tolerance slider from 28/56 to 39/49 and decrease the range (depending on the scene), a balance can be achieved and the house regains it’s purple colour and the purple fringe around the trees has been removed.