Thanks a lot everybody !

Thank you all for the clarifying information! The problem has been solved using the command recommended by Pablo. All my attempts to clear the Windows font cache have failed, except for this one:

 mtxrun --script cache --erase && mtxrun --generate

I started by copying the fonts into the \texmf-fontes\data\ directory, then I emptied the cache. Then, the following command:

 mtxrun --script fonts --list --pattern=didot --all 

works and correctly returns otf and ttf theano and gfs fonts.

And thanks to Andres : your MWE compiles perfectly !

Now, I must confess that I don't know why installing new fonts in c:\windows\fonts, even after laboriously purging the Windows font cache and reloading the CTX font cache, didn't have the expected effects ...

I did a test with Windows 10. 

Installed a fresh copy from the garden, Got the GFS Didot font from Google Fonts, and installed it system-wide (C:\Windows\fonts). After that, I ran 

mtxrun --script fonts --reload

Then tried the pattern suggested but did not work. However, then I tried a pattern without asterisks:

mtxrun --script fonts --list --pattern=didot --all

This returned the font: 

C:\griego>mtxrun --script fonts --list --pattern=didot --all
identifier           familyname    fontname             filename                                   subfont   instances

gfsdidot             gfsdidot      gfsdidotregular      c:/windows/fonts/GFSDidot-Regular.ttf
gfsdidotnormal       gfsdidot      gfsdidotregular      c:/windows/fonts/GFSDidot-Regular.ttf
gfsdidotregular      gfsdidot      gfsdidotregular      c:/windows/fonts/GFSDidot-Regular.ttf
theanodidot          theanodidot   theanodidotregular   c:/windows/fonts/TheanoDidot-Regular.ttf
theanodidotnormal    theanodidot   theanodidotregular   c:/windows/fonts/TheanoDidot-Regular.ttf
theanodidotregular   theanodidot   theanodidotregular   c:/windows/fonts/TheanoDidot-Regular.ttf

trying to render Pablo's example didn't work at first, but after i called the fonts with the name returned by mtxrun:

\mainlanguage[agr]
\definefontfamily[mainface][rm][gfsdidot]
\definefontfamily[mainface][ss][theanodidot]
\setupbodyfont[mainface]
\starttext
GFS Didot:

\doloopoverlist{\tf, \it, \bf, \bi}
  {\recursestring\input aristotle-grc\par}

\ss
Theano Didot:

\input aristotle-grc
\stoptext

Everything compiles correctly, and the font is used. Since it's only a single font file with no variants, you don´t get italics, bold etc. But the font is being used. 

--

Andrés Conrado Montoya
Andi Kú
andresconrado@gmail.com
http://sesentaycuatro.com

http://messier87.com

http://chiquitico.org

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