Is one of accurate registration and joint callibration of the different sensors. Subtle errors in this lead to all attempts at composite (i.e. colour) images having distracting colour bands that occur in slow waves across the images.

See, for example, this "cydonia" image

Note that the waves of chromatic aberation are stronger than the natural colour variations across the scene. Now it happens that this is an IR multispectral image, but similar problems apply in colour.

For the Themis camera team there are two reasons why they don't want to release too many pictures like this:

1) They are not "scientifically" or even aesthetically accurate

2) It makes them and the camera look as if they are performing badly.

Until they crack this problem, colour images are likely to be few and far between, until the full data release date when everything will be thrown to the wolves..