The Meaning of Life, the Universe, and Everything.
In minecraft 1.9, Mojang introduced the potion effect called Glowing. It allowed the player to have an outline. You can color glowing, but even I don't know how. I suggest you should be able to be able to glow, for example, blue, like this: /effect <player or entity selector> glowing <how long the effect lasts> <the level of the effect> blue. Detailed glowing is my second idea. You know how the charged creeper has that cool lightning effect around him?
Detailed glowing would allow a player to get that effect, and other things like that, with this command: /effect <player or entity selector> glowing <how long the effect lasts> <the level of the effect> creeper_glow, for example. creeper_glow gives you the effect a charged creeper has around it's body.