Rats come in a variety of different colours, from the standard albino lab rat to the brown or black wild rat or pet store "hoodie." However, there are many less common colours, including fawn, blue and mink. These colours are the result of careful breeding, and are unlikely to appear randomly in the population. Unusually coloured rats act just like their more common cousins, but are normally available only from speciality breeders.
Chocolate rats have even, dark brown fur with no ticking, which distinguishes them from the wild brown "agouti" colouration. These rats have black eyes normally, but dilute genes lighten the colour to cocoa and produce ruby eyes in some individuals. Cocoa rats are also evenly brown, but lighter in tone. Chocolate and cocoa are both produced by the b/b gene. According to Spoiled Ratten Rattery, when chocolate combines with blue, the result is a "classic lilac" -- similar to the lilac produced from American mink, but a pinkish dove grey colour with dark ruby or black eyes.
Known as topaz in the United Kingdom and Australia, this agouti-derived colour comes from a dilute red mutation (r/r). These rats are golden to orange with red eyes and a bluish-grey belly and undercoat. A related colour, amber, produces a golden fawn rat with red eyes and a cream-coloured belly. In non-agouti rats, the fawn mutation produces a beige coat. This colour is more common in pet stores than fawn.
Mink rats are non-agouti rats with a greyish brown coat and a slightly blue tint. They have black eyes. A lighter variation of this genetic mutation is called lilac. This rat is lighter in colour and has pink tones to its fur. A separate mutation causes Australian mink. This similar-looking coat colour is silvery brown, and can vary from bluish to warm grey. Australian mink rats often have silvered fur with white flecks. When the mink mutation appears in agouti rats, the result is a cinnamon: a warm russet rat with darker guard hairs and reddish ticking.
Blue rats are actually a bluish-grey colour, similar to Russian blue cats. Some are dark slate blue, while others are a light, almost silver colour. Lighter blue rats usually have ruby eyes. Despite their similar appearance, the two main types of blue rat -- Russian blue and American blue -- are actually caused by different genetic mutations. Russian blue rats are black rats with a dilute mutation known as d/d, while American blue rats have a dilute g/g mutation. American blue can combine with mink to produce an extremely pale rat with ruby eyes, also known as a platinum rat. Russian blue combines with agouti to produce a ticked, bluish rat with black eyes and a silver belly.