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More than a month has passed since Dawn's arrival to Ceres but we have not seen a single colour image of it in natural colors. Why is this so?
Clarification.
I am not interested in the details on how space imaging is working in general. I am interested to know why Dawn does not produce color images of Ceres so far unlike other spacecrafts (for instance, Cassini, Messenger, Voyagers, Pioneer). Even New Horizons already took a color image of Pluto.
The Dawn spacecraft is carrying 3 electromagnetic instruments; the Framing Camera (FC), the Gamma Ray and Neutron Detector (GRND), and the Visible and Infrared Mapping Spectrometer (VIMS). Your question is presumably about colour images in the visual spectrum and not false-color images of wavelengths not visible to humans, so let's focus on the Framing Camera.
Ceres as seen by the Dawn orbiter (Courtesy NASA)
The Framing Camera is designed to acquire detailed optical images for scientific purposes as well as for navigation in the vicinities of Vesta and Ceres. Dawn carries two identical and physically separate cameras for redundancy, each with its own optics, electronics and structure. Each camera is equipped with an f/7.9 refractive optical system with a focal length of 150mm and can use 7 color filters.
The camera on the Dawn orbiter, and most cameras in space, do not have the kind of image sensor a regular digital camera has. In this fascinating article on the Planetary Society website, Emily Lakdawalla writes:
Space cameras usually have detectors that can detect light across slightly more of the electromagnetic spectrum than human eyes can see, from near-ultraviolet to near-infrared wavelengths. More importantly, while the pictures that they take are often pretty, they are not intended to be for snapshots; they are sophisticated, precise scientific measuring devices.
The FC is more of a 'photon measuring device' than a conventional camera. It measures the numbers of photons that land on each pixel of the sensor, not their wavelengths. However, it can perceive photons of different colours using filters.
A filter is like a gel on a theater light. It turns white light into colored light. It does this by limiting the wavelengths that can pass through the filter. Dawn's Framing Camera has eight different filters fairly evenly spaced across the part of the spectrum that its CCD is capable of detecting.
A graph showing the different wavelengths of visual light each filter (numbered F1-F8) allows through. (Courtesy CERN)
The image sensors in ordinary consumer cameras here on Earth actually also record light according to its intensity in three separate colours and then process that information to create a full-colour image. Different parts of the sensor are designed to respond only to certain wavelengths of light - some in red, some in green, some in blue. There are no sensors that can detect what wavelength of light is hitting them, they only count how many photons hit. One way or another, all cameras combine information gathered separately about light intensity at different wavelengths, and then combine that into a colour image. In fact, your eyes do this too.
Essentially, Dawn takes 'black-and-white' photographs at a very high resolution with each filter. Hence around eight images are created, each showing the raw photon-measurement in that respective section of the visual spectrum. These images can then be used to pretty accurately colourise the surface of Ceres and produce 'colour images'. However these would still be classified as 'false colour' pictures.
Why haven't any colour pictures of Ceres been released by Dawn? The answer is that they simply haven't been made yet. You can access the latest raw images from the Framing Camera here. NASA will presumably create colour pictures from the raw data in the future, and so could I, and so does @TildalWave. The colour these sorts of images are created on Earth, by people, not by the probe itself. There's no real reason to colourise an image other than beauty, the scientific data is gathered from the RAW file itself.
Further Reading:
CERN document on the Framing Camera.
The sensory equipment aboard deep-space satellites tend to be very different from commonly encountered analogs. In the case of Dawn, it has 2 framing cameras capable of capturing visible and near-infrared light using 7 color filters. The goal is to combine the color filtered images with data from the mapping spectrometer to analyze surface mineral composition. The storage capacity of each instrument and transmission bandwidth to Earth are both limited, and researchers are working under significant time constraints, so producing photorealistic images is not a high priority task.
Additionally, as userLTK's article pointed out, the color differences on the surface are subtle, so map rendering is mostly done to make identifying differences in surface composition easier.
There are some color pictures. The first article below indicates the early pictures were black and white but they now have a color map of the planet.
http://www.cnet.com/news/dwarf-planet-ceres-reveals-surface-secrets-in-nasa-color-map/
and from the NASA website: http://www.nasa.gov/jpl/pia19063/dawns-first-color-map-of-ceres/#.VXfE9kYTDW4
Edit: as pointed out in the other answers, I should mention that the color in the photos in these articles is enhanced. Ceres isn't that colorful.
Combining the data from the different filters into a color image is complicated by the fact that because the filter has to be changed before each image is taken, the probe moves enough while that happens that the perspective in the next image isn't quite the same.
To get the color image, several frames, from filters for different wavelengths of light, have to be aligned and then combined. This is tricky because the perspective has changed, so some manipulation of the image has to occur to try to compensate for that as best as possible. It takes time, and if the image that would come from it is of no scientific interest, and wouldn't be pretty enough to be interesting to the public, it is just a low priority.
