Marc, colour reproduction is both a fact and personal opinion. Let me explain the first point in my own experience.
Photography of pigmented lesions of the face requies lenses with accurate colour reproduction. We always use studio flash to do the photography. We send both the negatives and Scandisks to the same lab for processing. We have took thousands of photos using various lenses including Minolta, Canon, Leica and Zeiss. Poor colour reproduction of lenses leads to images of poor diagnostic informations. We compare the prints directly with the faces of our patients under white light. In the end,we found that Zeiss in general,and in fact always,produces images with good colour reproduction and hence good diagnostic informations.
For artistic considerations, people could have different opinions concerning the colour reproduction of lenses. For clinical evaluations,we exclusively uses Zeiss for its leading role in its accuracy of colour reproduction.
Although some,like Marc,would say Leica is more natural in colour reproduction, Leica lenses lose more colour informations than Zeiss in our opinion,but it does not affect its status for artistic use. Zeiss is very accurate in colour tones and to some people it becomes too vivid.