[NTG-context] convert color to black and white documents

Dear all, I am working on a text with plenty of figures, using modes to create both a screen and a print version. For the print version, which will be printed in black and white, I'd like to have all colors (including those of the figures) to be converted to grayscale so that I can softproof the document. Is there a way to do that in ConTeXt? I realize that these are multiple problems, addressing the conversion of the figures as well as the conversion of colored texts and other decorations that are done within ConTeXt. Thanks for any hints, Matthias