When creating a listbox item, the default value of the property SelectedBackgroundColorNumber for black is set to 16, where the SelectedBackgroundColor above it shows 0,0,0, which is incorrect. Selecting the SelectedBackgroundColor property and hitting Enter on the 0,0,0 value changes SelectedBackgroundColorNumber to 0, which makes the text transparent. So the two properties don't seem to share the parallel color index.
In general, the SelectedBackgroundColor shows the color in its RGB form, where the SelectedBackgroundColorNumber proabaly shows them in index color numbers, and the higher numbers in it don't make any sense.