This is an issue I mainly have in PvP. I find that when I'm playing, say, HB, and I target someone that's clustered in a group of 4+, I tend to get juked and lose who I'm targeting. It happens often enough that when I'm in the thick of it, my target could take off and it would take me longer than I'd like to identify them again.
Of course, we have the option to declutter the IU a little bit by turning off nameplates, but, personally, I like to have them on. To combat this, I feel like an option to change the colour of the nameplate of the person you are targeting would be extremely helpful.
Here's a picture to demonstrate how effective this could be.