There are four main body colours in most fancy guppies: the golden, blond, albino, and the original, wild, gray. There are also a couple of rare colours which are bronze (such as the Moscow Purple strain) and blue.
The dazzling colour shown in male guppies is produced by light-reflecting crystals of guanin under the skin or by skin pigmentation. Even in male fry which have not yet gained full colouration, this iridescent look can help distinguish them from the females.
The Golden Guppy
...or gold-bodied guppy is originally the result of a mutant gray guppy in guppy history, which had greatly reduced black pigment by 50%. This uncovered the hidden yellow pigment cells. The golden guppy also appears rather transparent.
The Blond Guppy
Another beautiful mutant, this guppy is similar to the golden guppy but is lighter and more translucent in colour. The blond guppy's actual number of melanin particles is nearly the same as the gray guppy's but they are much smaller, and they cannot disperse their particles the way gray fish can. If you cross a blond guppy with a gray, expect gray Guppies. Blonds are generally not as strong as grays, yet blonds are much more hardy than Albinos.
The Albino Guppy
The 1940's brought about the albino guppy mutant. By definition, these fish have no melanin pigment at all, which accounts also for their pink eyes. When breeding them there are often problems with health, fertility, good body size, and colour. If you cross an albino with a golden guppy, expect gray offspring.
Chameleons
The skin of a female grey guppy under a microscope will show many hundreds of small black pigments cells called melanophores, which contain melanin. Melanin is a black substance that can concentrate itself in the cell or disperse throughout it. This movement of melanin is dependent on the physiological state of the fish. For instance, if a fish is frightened, mating, or stressed, it may become darker or lighter in appearance. Most noticeable in a female guppy is the darkening of the eyes. This seems to be a sign of an unsuitable conditions or fright.
A guppy hovering over a white sandy bottom will appear pale and match the scenery much like a chameleon. Lightening of body colour is also a result of a sudden change in pH, (alkalinity vs. acidity), temperature, or salinity. This change may last for hours or even days. In these cases, the melanin has simply concentrated itself in the centre of the cells. When the fish is resting above a dark bottom, the fish's eyes sense this and send information to cause the melanin to disperse in the melanophores, which will cause the fish to appear dark.
Changes in colour in a male guppy is also a result of different water conditions. Although this ability is limited, it can be a problem when a well-bred guppy is shipped from one area to another, or when sent to a guppy show. The fish's colour can be remarkably different (such as blue changing to green) as a result of changes in water parameters.