It is also the longest of the four leads. This lead will be treated as the “common” pin of the RGB LED. As said, the circuit is built around CD 4029 IC and a few other external components. The ...

[Stefan] decided to recreate the 56×208 single color 2mm dot pitch display with an 128 x 64 P2 RGB LED screen respecting the same 2 mm pitch. The display is driven by an ESP32 DMA RGB LED matrix ...

To achieve this he created Fade, a custom programming language/framework for controlling LED animations from the ESP32. Fade is written for addressable RGB LEDs like the Neopixel/WS2812.