In the meantime your can set the color attribute of a specific annunciator cell after the RTMultiValueAnnuciator has been created.
ChartAttribute []attribArray = {attrib0, attrib1, attrib2, attrib3, attrib4, attrib5};
RTMultiValueAnnunciator annunciator = new RTMultiValueAnnunciator(pTransform1, processVarArray, numcols, numrows, attribArray);
annunciator.SetSegmentAttributes(0, attrib0);
annunciator.SetSegmentAttributes(1, attrib1);
annunciator.SetSegmentAttributes(2, attrib2);
annunciator.SetSegmentAttributes(3, attrib3);
annunciator.SetSegmentAttributes(4, attrib4);
annunciator.SetSegmentAttributes(5, attrib5);