well, the system palette only really exists in 256 color mode afaik
(15, 16, 24, and 32 bit modes don't use palettes), and depends on what
you are running. usually, afaik, windows apps don't really access this
palette anyways (and it is irrelevant outside of 256 color mode
anyways).
there is another, more or less constant palette, which can be found
somewhere, but I am not sure where exactly right now.
note also that Java is designed specifically to avoid OS, and
especially video mode, dependencies, which are implied by this kind of
thing.
so one question here is, why do you need it?