I reckon you'd need to keep an array of colours that the area uses. Search the area the mouth was last detected in, outwards, since this might be quicker to scan and find what your after. It's all down to the colours i'd say. Just search for areas of colour that match the ones first recorded, and try and match the rough size. You'll probably need to change the size every frame and maybe the colour values, as light may affect the person's head rotating etc etc.