In SPEED and DUPLX mode on a Cisco switch, what do the port LED colours mean?

SPEED: off = 10 Mbps, green = 100 Mbps, blinking green = 1000 Mbps. DUPLX: off = half-duplex, green = full-duplex.

Both modes reuse the same row of per-port LEDs; the Mode button decides which of the two you are currently reading. The SPEED encoding is ordered on purpose — off, then green, then blinking green as the rate climbs — so "darker means slower" is the whole mnemonic.

That ordering is what makes the panel useful at a glance: a rack of gigabit access ports should light up as a uniform row of blinking green, so a single steady-green or dark port jumps out as a link that negotiated down to 100 or 10 Mbps. Flip to DUPLX and the same trick finds the port that fell back to half-duplex — the classic symptom of a failed autonegotiation with a fixed-speed neighbour.