I would do it in three stages (note a higher resolution image would have been better to demonstrate but hopefully you get the idea)
1. Make the image a smart object and use the camera raw filter to correct the underexposure and increase the satruration


2. Add a local adjustment brush in camera raw filter and increase the exposure in the shadows and reduce saturation around the mouth


3. Add a new layer and clone some of the whiter fur around the mouth. Set the blending mode of this new layer to luminosity and turn the opacity down till it looks right


I hope that helps you
Dave