the problem is "i read too much" ...

yes, i read too much about probably old gimp versions which included informations in images (or something like that) and modified the color (don't ask me why !!!);
but i found the assertion that gimp 2.8 had eliminated this problem;
and i searched another origin;
in fact the problem was about drupal and rga in views/grid : a component was at 255 instead of 0 (fresh installation without modification) and it modified my color ...

excuse-me Mr Gimp ;-)