Highlight-able entities

I want to be able to highlight specific entities that the player is looking at, i.e.:

I have a text display that I want to change color/become bold or whatever when the player is looking at it
I am going to have a character select area with armor stands showing different characters, with a clickable text display below to select them. I would like the armor stands to be given the glowing effect when the player looks at the text display underneath the armor stand (each one will probably be tagged with the character name)

Probably have this controlled by a "highlightable" tag or something so I can easily add more highlightable stuff down the line if need be