On this video tutorial Michael Woloszynowicz (from vibrantshot) takes a look at how to fix mismatch skin tones in an image in Adobe Photoshop.
Woloszynowicz uses solid fill layers and tonal averaging as well as some luminosity correction depending on your image. Woloszynowicz shows two different methods for doing the tonal fixing on two different images. Keep in mind that this method requires Photoshop CS5 and higher.
On the video Woloszynowicz mentions a free Photoshop action for doing some of the work which takes several stages – Click here to download the action.