Ross,

Are you saying that the nitrogen-frozen specimens maintain their color even
after subsequent thawing and drying?  Or just that they retain their color
as long as they remain frozen?

I  work in a group of labs which provide, among other technologies, a full
range of histological services, and subsequently I frequently must
quick-freeze tissues of various kinds.  While nothing commonly available
will freeze tissues as fast as liquid nitrogen (-196 degrees C), a couple of
other methods will freeze tissues pretty rapidly.  One is immersion in a
liquid which has been cooled with dry ice (-78 degrees C).  Obviously a
liquid must be used which has a freezing point lower than -78.  Isopentane
works best, but acetone works well and ethanol can also be used. Another
option is one of the commercially available aerosol refrigerant sprays
(tetrafluoroethane or similar compounds) which are available in small, easy
to use cans, and which can bring a tissue sample down to about -50 degrees C
in a couple of seconds.

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