These are the dragons that rule the Draco Constellation. They are of different colours to differentiate their abilities, such as fire-breathing, ice-breathing, etc. Each of them have control over one system but there is massive in-fighting between the dragons to take control over more and ultimately to displace Aerie - the oldest dragon. No one knows where the dragons come from but they are definitely a precursor of Earth's dinosaurs. Goodness knows were that may lead to in future stories. In the system of Grumium one of the last Earth groups to be transported through the portals was a group of archaeologists who are searching for something. You can go over to the Stories page and find out what Professor Maximus Smyth-Barton and his team are doing.

As a reminder here is the constellation map again so you should be able to get your bearings.