[NTG-context] Metapost and CMYK

Hi all, I have some questions about cmyk in combination with Metapost. Just by using \setupcolors[state=start, cmyk=yes, mpcmyk=yes, rgb=no, ] I get cmy(k=0) graphics, although all used colors are still defined in the rgb color space. This is really a nice feature and the automatic color conversion works relative good. Only (so far I have seen) gray tones have a noticeable tendency to brown (Acroreader 7.09). I have tested one of those gray tones by defining it as cmyk and the result looks ok. RGB(223,223,227) CMY(0.12548, 0.12547, 0.10980) CMYK(0.01762, 0.01760, 0, 0.10980) So is there a way to get cymk instead of cmy? Peter