What is your system for tagging players?

I sort of do a stop light approach with green/yellow/red with some other colors added in if they are available for that site.

Green = fish
Yellow = reg of some kind
Red = good reg (reasonably aggressive, somewhat balanced, reasonable call/fold, etc.)

light blue = positionally aware fish or fish-reg (lots of leaks, but knows position)
dark blue = tight reg or tight fish (like <20vpip)

purple/pink = VIP, mega whale, 60vpip+, preflop jams, erratic play, etc.

orange = between yellow/red

I use a pretty simplified one because I play on ignition so I usually don't have many hands on players.

Yellow= fish
Purple= aggro fish
Green= whale
Red= reg
Blue= non exploitable fish or reg fish/tight fish

Purple: Insanely out of line (50+ vpip)
Red: confirmed fish
Pink: thight fish (30 to 35 vpip, i consider that thight for fish standards, not worth seating if to my direct left)
Orange: reg non descript
Green: Reg thats likely better than average, seen do something good (4b blufffing, donking in a spot that made sense etc)
Blue: particularly thight reg

I usualy try to identify the players that are passive, overly aggressive, and stations. I don't really tag players based on vpip, only on post flop play or wether they limp (shows that they are probably passive and possibly stations)

I think in big micro pools where there are basically no good players the edge gained from tagging is marginal. I do mark very spewy players, but understand that they could easily turn into nits overnight.

Yellow: rec
Red: aggressive, mostly rec
dark green: good people who donate money to the table
light green: reg
purple: crusher

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Heard this advice from Uri Peleg some time ago... Don't tag anyone as good Reg, there are no good Reg until you get to high stakes.

It's all relative and subjective, so while he might not find someone as a good reg, someone else might believe them to be and poker is a game where it is the relative skill that matters and not the absolute skill.