I think that I have found a bug in the graphics 2.0 daily builds (Build: 2013.2004)
If I try to change the default background color using
display.setDefault("key", r, g, b, alpha)
then the screen turns white no matter which values I choose except for 255, 255, 255 and 0, 0, 0 .
The alpha value seems to be working. There is no error message or something else, the background is just white.