In the grim darkness of the far future there is only war! Humanity is pitted against hordes of hostile aliens. It is not only external threats that man must face. The greatest threat to the Imperium is from within. Always are the forces of Chaos seeking to gain a foothold in the hearts of men. A great civil war waged between the Emperor and his favorite son Horus when he fell to the evilness of Chaos. The Emperor himself had to defeat Horus and in doing so it almost cost him his life. Now the Emperor is kept alive by ancient technologies. The Astartes are the elite warriors sworn to protect the Emperor at all costs. The Imperial Guard is the hammer of the Emperor, entire planet systems dedicated solely to producing guardsmen. Without them the Imperium would have be destroyed long ago.
Warhammer 40,000, more commonly known as Warhammer for 40k or just 40k, is a table top war game in which a player assembles his or her army and pits it against other players. A player is able to choose from many different armies. In a sense the Imperium of man is the "good guys" but due to the hostile environment of the 40k universe there are no truly "good guys". The Imperium rules with an iron fist, no dissension is tolerated. The rest of the armies in the 40k universe range from pure evil, Chaos, to the honorable Tua Empire.
It is the responsibility of the player to assemble and paint their army. The color scheme of the army is entirely independent to the player's discretion. Some armies do have a preset color scheme such as the Empire's Children, Ultramarines, Cadians, and many more. It is the player's choose to pick one of those armies or to design their own.