class DynColor{
   struct _combo{
       Color c;
       char  f[4];
   };

   operator Color()const{
      switch(f[3] & 3){
      case 0: // the 64 bits are a Color*
          return * reinterpret_cast<const Color*>(this);
      case 1: // a normal color hold at data.c
          return c;
      case 2: // Blend, with f[0], f[1] the index
              // of System colors, hopefully
              // we don't have more than 256 of them
              // , and f[2] hold the third paramenter of Blend
         return Blend(GetSysColorFromIndex(f[0],
                  GetSysColorFromIndex(f[1]),
                  f[2]);
       case 3: // let's handle the
//ctrl.Add(AttrText("Hello there!").Ink(IsDarkTheme() ? Blend(White(), LtBlue()) : Blue());  
               // case here
            // theme dependant blend
           return IsDarkTheme() ?
                Blend( treat_c as byte[4], and store blend information there) :
                Blend(GetSysColorFromIndex(f[0],
                  GetSysColorFromIndex(f[1]),
                  f[2]);
          
      }
   }

private:
   combo data;
};

ctrl.Add(AttrText("Hello there!").Ink(ConstructDynColor(DarkBlender{...}, NormalBlender{...}));