Having solved the page colour issue, I have now detected that Moodle 2.4 uses the Moodle exercise title to replace the original Hot Potatoes exercise title.
Of course, there is no particular reason why the two titles should be different, except
a) an image inserted in the title field does not appear;
b) the font size cannot be altered to match what follows (minor aesthetic problem) whereas the specified title colour is indeed applied;
c) the title cannot be deleted (to be replaced by an image, for example).
Could you help?