A colony is a new planet, similar to a player's homeworld, created by a Gaia Colony Ship. Colonies share crew points and research with their homeworld. A player may have up to eight colonies in addition to their homeworld.
A new colony has a random number of fields assigned when it is colonized as described here. A user can abandon a planet and then recolonize it to possibly change the number of fields on the planet. The planets with the highest potential field slots available are those located in slots 4, 5, and 6. It is recommended to keep colonies with 200 or more field slots.
Note: The Gaia Colony Ship is automatically destroyed in the process of creating a Colony. If a new colony is created, any ships sent with the Gaia Class will be placed in orbit and any resources will be deposited into the mines. If someone manages to colonize a slot before your colonization mission arrives, your fleet (including the Gaia) will return to the world it was sent from.
There are two ways to send a colonization mission: (1) From the Galaxy Screen, you may simply click "colonize" wherever you see an empty planet slot; and (2) From the Fleets Screen by selecting a Gaia Colony Ship (plus any accompanying ships), putting in the coordinates you want to send to, making sure the "Colonize" mission type is selected, and hitting "Dispatch".
New players often build their colonies within a few systems of each other. This can be helpful for pooling resources and mutual defense. However, it also limits the pool of potential targets. Distributing your colonies over a wide area enables you to easily reach more systems, thus enlarging your potential target pool.