Xeelee Sequence (series of novel)

The Xeelee Sequence is a series of novels by the British science-fiction writer Stephen Baxter. Mostly set in the very far future (tens or hundreds of thousands of years) the series follows a humanity traumatized by a brutal alien occupation early in its history as it attempts to conquer the galaxy and exterminate all other intelligent life. Typically of Baxter's work, it is not especially cheerful.

The 2004 novel "Exultant" takes place about twenty-five thousand years from now, near the end of the "Third Expansion of Mankind," overseen by the Interim Coalition of Governance. The ironically-named Interim Coalition (in power for ~20,000 of those years) has almost completed its genocidal project and purged the whole of the Milky Way; only one enemy remains, the incredibly advanced, near-godlike Xeelee, unassailably entrenched around a supermassive black hole in the galactic core.
Eugene Ipavec, 10 August 2009

by Eugene Ipavec, 10 August 2009

by Eugene Ipavec, 10 August 2009

by Eugene Ipavec, 10 August 2009

by Eugene Ipavec, 10 August 2009

by Eugene Ipavec, 10 August 2009

Despite the fact that the novel visits the Coalition's very heart on Earth, no real flags are mentioned. The flag of the Coalition is however presented in a holographic form when a VIP - a member of the Commission for Historical Truth, the Coalition's ideological branch - visits a frontline base and is greeted by a troop review:

"Virtual flags, adorned with the green tetrahedral sigil of free mankind, began to ripple in a nonexistent breeze." (pg. 389, Random House/Del Rey first hardback ed., 2004)

The "tetrahedral sigil" is mentioned often in the book. Green is the color of humanity in the series: the Coalition's military is named the Green Army, and military spacecraft are called "greenships," for example. The color is apparently associated with what was lost during the Qax occupation of Earth, which saw the planet's entire biosphere deliberately destroyed and replaced with nanotechnological synthetics as a morale-breaking measure.

No hint is given of the background color. The Commission for Historical Truth is the only other body given a distinguishing color (black), though as 95% of the book's human population lives in free space, it would not be an out-of-place choice anyway. Any number of reconstructions are possible:
Eugene Ipavec, 10 August 2009