I have also noticed, that many winning players wears blueish outfit, just to make outline even less visible. I consider it cheating.
Ban all blue textures, immediately!
(shaking head so hard...)
Those red outlines are for people who barely learned how to play any shooter game, to help identify what moving objects are most probably enemies and what are not.
You should be able to easily distinguish an object moving against a static background without it being brightly coloured and pointed out to you "hey look look it's an enemy".
Some teams use intentionally blue shaders. I have no problems to spot moving object. But If I need to spend ANY time to tell if it is my team or enemy, that's unfair. I need to shoot immediately, not to think "is it blue guy with red outline?", "Is that red glowing guy enemy, or just doomguy rushing?", "Enemy has a Quad? My announcer does not tell that yet... Still announcing 5 medals for perfect, excellent, revenge shoot from the grave"...
That's enemy team. With outlines they still looks more blue than all in my team.