discolored hydrocal castings

I have been pouring hydrocal in urethane molds and have consistently
had problems with slight discoloration in the castings. The
discoloration is grey, sort of splotchy, sometimes a little cloudy,
altogether annoying. Am I wrong to assume that hydrocal should always
cast white? I've been using white dish soap for my release. I
lightly mist the mold with an fine-spray atomizer. I've tried a
number of different pouring strategies thinking that maybe my pour
rate was too fast. I've mixed the hydrocal by hand and with a mixer,
thinking that the mix was the problem. I've called the techs at
smooth-on and at poly-tek looking for answers. Could it be my water?
Is the urethane not entirely mixed in areas. It seems completely
cured. Should I give up on this mold and start over? Has anyone out
there had similar problems? I just want a nice white finish. No
fuss, no muss. The material is what it is. Looking for answers from
plaster/gypsum masters. Any help is greatly appreciated.