I think I may have encountered a set of related bugs in my (trial) version of Dorico Pro 2:
When in Write or Engrave mode, changing a time signature’s color in the Common group of the Properties panel does not appear to have any visible effect (apart from the small colored square next to the “Color” property changing as expected). The same holds true (mutatis mutandis) for key signatures, bar lines, stems (Engrave mode only), and probably more.
Also note that unselecting and reselecting a stem after attempting to change its color results in the aforementioned small colored square vanishing (and being reset to black when toggling the “Color” property off and on again).
When in Engrave mode, attempting to change an accidental’s color results in a change not of the selected accidental but of the notehead it precedes.
(In case this behavior is intentional to some degree, I find it to be rather confusing.)