With NXG style buttons, you can specify colors for rollover.
The property
is actually a 6-element array.
When you write to it, element [4] sets the rollover colors for the FALSE state, and element [5] sets the rollover colors for the TRUE state.
But when you read from it, elements [4] and [5] come back as all black (which is inaccurate, the colors that you've written do stick).