What follows are two design challenges. Please think about them and
suggest a solution.

1- Please take a look at the "neopixel" devices (WS2812B or
equivalent) and think about the best way to drive a neopixel string
from a microcontroller.  With or without looking at existing code out
there (without looking is more fun, and often more effective too).

2- Imagine you have two microcontroller boards with 3 RGB leds each.
They look like a traffic light, so you want to write a traffic light
application: You want one device to be red while the other is green
and so on.  How can you achieve that with one wire only between them?
We want the same firmware image in both devices, and we can assume
they are powered on at the same time (we can solve anyways, but it's
more difficult.  If possible, connect the same GPIO pin (i.e. pin X of
the first board il shorted the same pin X of the second board).