"Some light contrast enhancements have been made to emphasise certain features and to bring out features in the shadowed areas. In reality, the comet is extremely dark ­– blacker than coal. The images, taken in black-and-white, are grey-scaled according to the relative brightness of the features observed, which depends on local illumination conditions, surface characteristics and composition of the given area."

http://www.esa.int/Our_Activities/Space_Science/Rosetta/High...

Probably B&W. Even color images (aka like you see from the MER mission) are just a B&W imager multiple exposures - each with a different color filter.