There has been some confusion among the names of dyes. Methyl
  green and light green are completely different in their uses.
  It would be possible to go on and on about this, but it would
  take up at least 4 screenfuls of text. All the information is
  there in the textbooks. Four points are, however, worth 
  emphasizing.

  1. Methyl green (CI 42585) was a lousy dye (always contaminated
     with crystal violet, which had to be extracted with chloroform) 
     and it is no longer sold. Dyes sold as "methyl green" are
     really ethyl green (CI 42590). If the catalogue doesn't say
     this, don't buy the stuff - they might just be trying to
     pass off ancient stocks of real methyl green. If you look
     in a good catalogue (Sigma, Aldrich, and I'm sure many others),
     the real identity of the dye will be clearly stated. It's up
     to us, the users, to start asking for ethyl green rather than
     methyl green.
  2. Ethyl/methyl green is a cationic dye used for staining nuclei
     by virtue of their DNA content. Used alone, it also stains
     other basophilic materials such as RNA, cartilage etc.
  3. Light green is an anionic dye, so it stains stuff like cytoplasm
     and collagen; also cellulose in plant tissues. Used as a
     counterstain it colours nearly everything. It doesn't pick out
     the nuclei of cells.
  4. Fast green FCF has the same properties and colour as light green
     and is a better dye for all purposes because it is less prone to
     fading in stored stained preparations. 

  Before using ethyl green, light green or fast green FCF as a
  counterstain for immunohistochemistry you must learn the general
  principles of staining with cationic or anionic dyes. If you don't
  you will make mistakes that could spoil valuable immunostained
  slides. 
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Exerpts from earlier exchanges follow.

> Wait a minute, wait a minute, my precious liver has other things to do. I
> read that methyl greens are pre-cleared in chloroform.  I did it once and it
> was a glorious mess, but it was clean afterwards.
> Dear Dana, can you tell me what light green is. I mostly now the german
> names and light green doesn´t ring a bell. What does it stain? Where do I
> get it from? How do I use it? I´m still looking for a non-nuclear
> counter-stain...
> > 
> > Most methly greens need to be pre-cleared in cold acetone and cold benzene,
> > woops there goes your liver, did you try working light green as a substitute?
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 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1