That makes no sense - the fourth colour could have been any in the ST palette. Choose appropriately for artistic reasons. Pick either a highlight colour (orange

That makes no sense - the fourth colour could have been any in the ST palette. Choose appropriately for artistic reasons. Pick either a highlight colour (orange as on the Amiga and whatever early Unix desktop I once used), or a complementary colour (maybe grey?). CGA was awful for many reasons, one of which was that all colours were fixed.