Is it possible to change the colour of the bar chart that the students see in the choice activity?
The default purple does not go with my site's theme
There is a $CFG setting, unfortunately i could not find it in the web administration or in the settings of the plugin.
So I put this in my config.php in the moodle root src directory:
$CFG->chart_colorset = array('#0067a5');
and restartet the webserver (nginx or apache) => it worked.
In this case it is dark blue ('#0067a5').