That's not it, as far as I know. I will take you to RED creatures UNDER lvl 15 that will ALWAYS attack you and if you are 6 lvls higher than those creatures, you will get NO xp!
So, RED = the creature will attack you, NO MATTER WHAT. If you are in it's range, it will attack, regardless of your lvl. You can be even 70. Test it.
Respect
There are two different 'reds' here. The name tag being red means it will attack...it is aggressive. The level of the creature is also color coded. When you look at a creature, on the bottom right hand corner you can see the level. That was the red that i was explaining.
It is a color code, just like the quests. Basically it is supposed to determine the likeliness that you will be able to take it down. A red quest, or a red monster is more difficult, almost impossible. For those who will argue that they have done a quest or killed a monster that was red in level...notice I said almost

The higher the level of the quest or the monster, the more xp. The color code again is- grey (no xp) green(a little xp) gold(more xp) orange(more more xp) and red (most xp)
Namaste