well, the ugly way (fmpov) would be to get all actors of class, select your fieldclassBP and change
colors within a foreach loop, in case you clicked one.
A proper way would be, you create a Blueprint (f.e. AFieldActorContainer) which contains an array of your FieldActors.
You spawn such a Container into the world and within its class it spawns your 3x3 field actor grid. Or you just make them components of the Container BP.
The advantage you have now, is that the container class knows about all of those fields.
And each field knows its parent (the container) in case they are components, if not you can inject the parent to your actors via a function.
So each field will have a click event function. And if it triggers the field sends a message to its parent, telling him that something needs to be done and
that is coloring all the fields inside the array to green.
You can even set up an Input Event (Keypress “R”) inside the container and iterate over the array again to set the color back to blue.