Alpha 0 should be transparent, 255 should be opaque, but in the code below 1 is transparent,
255 is opaque and 0 is also opaque.

void setup()
{
size( 100, 100 );
background( 255 );
fill( color( 0, 0, 0, 0 ) );
textFont( loadFont( "Georgia-48.vlw" ), 24 );
fill( color( 0, 0, 0, 0 ) );
text( "0", 0, 0, 50, 50 );
fill( color( 0, 0, 0, 1 ) );
text( "1", 0, 50, 50, 50 );
fill( color( 0, 0, 0, 64 ) );
text( "64", 50, 0, 50, 50 );
fill( color( 0, 0, 0, 127 ) );
text( "127", 50, 50, 50, 50 );
}

verified.. not sure what's up. use fill(0, 0, 0, 0) in the meantime, since
that seems to work correctly.

the reason this doesn't work is that color(0, 0, 0, 0) creates an int that
is simply '0'. so with fill(0), that evaluates to black. this is a problem
of 'color' not being a real type, but just an int, plus the fact that we
overload fill() to use both int/color for a color, and also an int for a gray.

there are multiple workarounds:

1) use fill(0, 0, 0, 0)
2) fill(c, 0) where c = color(0, 0, 0);
3) color almostTransparent = color(0, 0, 0, 1);
4) color almostBlack = color(1, 1, 1, 0);

one of these workarounds should work for most any situation.

the root cause is the same problem behind bug #432.