Change the name of the entity you are looking at to anything you want, like shown below. The name will always be visible, not just when you are looking at the entity.

Commands

/entityname <text with color/formatting>
/entityname gradient <color1,color2,...> <text>
/entityname rainbow <text>

Rules

• Don't sell the ability to use this command to players for ingame money/items
• Don't use it too much for other players, even for free.

Additional information

• Use /colors to view a list of color/formatting codes that you can use.
• You may change any entity's name as often as you want