In the beginning of the game, the general trinket choice is fairly standard, and almost everyone except for possibly the jungle or support goes for the yellow trinket.
Beginning Trinket Choices
Mid Lane: Yellow Trinket
Top Lane: Yellow Trinket
ADC: Yellow Trinket
Support: Yellow/Red Trinket
Jungle: Yellow/Red Trinket
Mid-Late game Transition
SupportAs the game transitions over to mid-game however, one mistake I often see supports make is forgetting to sell their yellow trinket and buy a red trinket after purchasing sightstone.
Every player may only place 3 stealth wards, including the trinket ward
The only reason that a support might want to keep their yellow ward is if they are planning on upgrading to pink ward. Even if this is the case, this should be done at the very very end, and ONLY if *someone* on your team has already upgraded their red trinket to a sweeper.
Needless to say, the occasion for a support to upgrade to the pink ward trinket is very rare.
AD CarryThis one's the one I really want to highlight on, since I almost NEVER see the AD carry do this properly in solo queue, and that is to switch from the yellow trinket to the BLUE trinket once team-fighting starts. Some may even consider getting it from the very beginning if you're against a "cheese" support like Blitzcrank, Leona, or Alistar.
As you can see from the picture above, even the basic blue trinket is enough to prevent the majority of facechecks into dangerous bushes. It also performs the basic activity that AD carries want yellow wards for, which is to keep enemy champions out of the fog of war.
Of course there are 60 second intervals in which a yellow ward will be more useful to plan sneak attacks, but those generally don't make up for the blue trinket's massive utility.
Use me! I'm awesome!
Everybody ElseJunglers almost always want to get the red trinket and upgrade in solo queue, as it helps them secure objectives and sweep for ganks more efficiently. As the objective leader, Junglers also understand which areas they want to ensure are clear prior to starting a dragon or a baron.
Top/Mid generally want to also get sweepers, and also possibly buy a green ward or two (remember, the support can only place three at once.) However, either top or mid may also want to upgrade their yellow trinket into a pink trinket instead depending on how full the support is in inventory space.
This ensures that you're not strapped for map control at the end of the game.
ConclusionHopefully this post taught you something that you didn't know before, or perhaps reiterated something that you forget to do from time to time. Personally, I know I tend to forget to swap to blue trinket since I don't AD carry as much, but could definitely also do with a helpful reminder from time to time. God knows the entire team with basic yellow trinkets isn't doing us any favors...
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