Thanks, Jamal! No, that wasn’t it.
It turns out I was just being clumsy figuring out the correct way to target the hover state of the menu items. Something like this works fine:
#site-navigation a:hover{
background-color: #00c;
}
Why the child theme’s colors.php file doesn’t override the parent theme’s equivalent file is still a mystery to me, but the question is irrelevant as long as I can override the parent theme via the child theme’s style.css. Problem solved. Sorry for the bother!