Pygame - Screen shows "broken" colors instead of background color


I am trying to make a screen for a game on Python/Pygame but it seems as the color doesn’t work. I tried different color variations and get no error when I run it, but the picture on the screen stays the same.

Does anybody has an idea what is wrong with this window?

BtwI’m using mac, if that is of any importance.

Many thanks in advance!

screen = pygame.display.set_mode( (600, 600) )
screen.fill( (255, 0, 0) )