I was wondering why every picture we saw on internet about Rosetta's landing on 67p just black and white ? Is there something i should know ?
True fact is that most of the images seen so far are taken from the navigation camera (NAVCAM) of the orbiter, Rosetta, and that takes black&white pictures.
Here is a very interesting article about how these pictures are taken (ironically it is on a NASA blog). They state that they show pretty much what you would see with a human eye from there, even thought the comet is actually all very dark. The white parts are due to the contrast with the very dark surroundings.
I'm not sure about the images received by the lander. They are captured by the CIVA cameras, where CIVA stands for Comet Infrared and Visible Analyser. So I guess they might show some infrared light not visible to the human eye, then processed to be in b&w. Unfortunately, I could not find a precise explanation on ESA's website so far. (here their page for Philae instrumentation).
Regular digital cameras color filter arrays that put the pixel sensors so that it only detects a particular color. Therefore, there is a trade-off, because without the filter array, you would have a significantly higher-resolution black-and-white image.
However, most scientific digital cameras work differently. Rather, the usual technique is to put filters over the entire image, and switch out the filters if a different color is required. Subsequent differently-filtered images can then be combined into one image. But there's still a trade-off: putting in filters decreases the dynamic range of the camera significantly. Additionally, this approach does not require one to use the usual red, green, blue scheme; e.g., and IR filter would allow one to emphasize different features.
It is very likely that they simply didn't bother to put color filters in the cameras. Why would they? The colors are probably rather drab on the comet anyway, and it unnecessarily increases the complexity of the cameras (which is already a quite complicated panoramic array).