Hi @Klaus_K, thank you for the reply and the sketch!
Just tried out your sketch and the RGB LEDs work fine on my new board !
After trying out and playing around with your sketch, I now understand that the 'old' board lights up all the colors at startup, only for an instant (i.e., red, green and blue LEDs, showing a white-ish color).
For the moment I'll continue with the new board (my plan was to work on tinyML projects with it), and close this thread. If by any chance I'm able to get the 'old' board to work again, I'll open another thread.