Agouti: a ticked coat that looks brownish grey, having black guard hairs and a lighter grey belly, the eyes are black. Fun fact: Agouti is the colour most often seen on wild rats.
Berkshire: a marking description, indicating a solid coloured rat with a white belly, often with white feet and tail tip.
Black: a dark glossy coat, chocolate to black in colour which is often ticked with silver, the eyes are black.
Double Rex gene: hairless rats are a coat variety characterized by varying levels of hair loss. They are bred from curly-coated Rexes and range from having areas of very short fur to being completely bare. They are genetically produced by breeding different combinations of the genes that cause Rex coats, and since Rex is a dominant trait, there only needs to be one Rex parent to produce curly Rex-coated offspring. When two copies of the trait are present, the genetics cause hairlessness and earning such rats the colloquial name, “Double-rex”.
Dumbo: a variety of rat with ears placed lower on the head, named for resemblance to Dumbo the elephant.
Ruby: Rats can have black, ruby or red eyes. Black and red eyes are immediately apparent, while ruby eyes often appear black until seen in the light.
Self: a single colour to the coat, with no white markings at all.
Siamese: typically a gradient of colour along the body, darkest at the base of the tail and nose just like in Siamese cats.
Topaz: a ticked coat in orange to ginger colours with a distinctive slate grey undercoat and ruby eyes.
(As usual, I used http://www.dapper.com.au/whatrat.htm#agouti as reference to make sure my markings and colour information was correct. I also used Wikipedia to make sure my explanation of the Double Rex Hairless situation was properly accurate.)