Just had the same problem, what I did was match the post-id body class to the the nav menu item id.
Below shows an example for the first item in the Services menu:
https://www.ifonevan.com/best-iphone-plan-unlimited-voice-text-4g-data-for-65month/
We match postid-846 body class to #menu-item-1196
// Body class has .postid-846
<body class="single single-post postid-846 single-format-standard">
// HTML continues...
// Nav menu has #menu-item-1196
<li id="menu-item-1196" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-1196">
<a href="/best-iphone-plan-unlimited-voice-text-4g-data-for-65month/">Unlimited iPhone plan $65/month!</a>
</li>
// Nav CSS
ul.nav li.current_page_item > a,
body.postid-846 #menu-item-1196 a {
color:#ff6600;
}