Um.
Ok, so I can read the error message.
I cannot understand why I'm getting it.
Image is defined with:

Drawable is defined with:
<palette disableTransparency="true"> <color>FFFFFF</color> <color>FF0000</color> <color>0000FF</color> <color>000000</color> </palette>
And buffered bitmap is defined as:
background = new Gfx.BufferedBitmap(
{:width=>dc.getWidth(),
:height=>dc.getHeight(),
:palette=> [
0xffffff,
0xff0000,
0x0000ff,
0
]
} );