Hooking into updates to an entity reference field within Parent node Let's say I have a Structure Content type called "Fruit", and it only has a title.

And I have another Structure Content type called "Color" which is a dropdown menu with a few colors in it.

And i have an "entity reference" for Color on Fruit. So when I go to add a new Fruit, I can input in the Fruit title field "Apple", and then choose from the Color dropdown menu "Red".

I have a hook for node_update() which triggers when I change the Title of the Fruit. But when I change the Color, this trigger is not executed.

Which hook should I use if I want to track the Color changing on a Fruit?

This is for Drupal 7. Thanks so much in advance for your time and insight.
You don't have a direct connection for those(the same way some entities work at least) so best approach would be to have a node_update for Color and then inside it to query (can use entityQuery) and see which fruits have that color, from here you can do your stuff.

the following pulled out of my head since I haven't touch D7 for over a year so here we go

function node_update($node){
  if($node['type'] == 'color'){
    $query = new EntityFieldQuery();
    $query-&gt;entityCondition('entity_type', 'node')
    -&gt;entityCondition('bundle', 'fruit')
    //value or target_id
    -&gt;fieldCondition('field_photo', 'value', $node['nid']); 

    $result = $query-&gt;execute();
  }
}