The Xmas Light Strand item was the first cosmetic to have a dynamic effect after the glowing eyes. They were released during the Christmas 2021 event as a reward from Holiday Crate 2s dropped during ZDC.
Rather than other dynamic cosmetics that glow, the Xmas Light Strand changes between two color states on a clock. It will switch slightly offbeat, so you can get green plus lime or red plus yellow combinations for a brief moment in between the main red plus green and yellow plus lime states.
Lights when red and green
Lights when lime and yellow
Animated look at the Xmas Light Strand