Currently, the Player Entity does not respect custom name-related EntityDataAccessors in Entity.
This is because the Player overrides methods such as shouldShowName & getDisplayName. 

This makes it so you cannot assign custom colors/text to player entity names without either hacking around the scoreboard system or using a separate entity riding on top of the player to mimic a nametag.

In general, it would be very beneficial for map makers/developers who use player entities on their maps and want to display custom text over the player, rather than relying on such hacks.