Display getting darker

Hello,

I'm using BlitzMax 1.09 Demo under MacOS 10.4 and something weird happens with my game. I first display a start menu where the colours are bright, but when I choose the "play" option the game starts and the colours get all dark, as if the screen brightness was tuned down. And then it stays dark, even when I get back to the menu. (Both fullscreen and window mode)

Any idea why this happens ?

Thanks,
Klaus