Why do carp change colour?

1 Light levels help...
"During the winter months the water temperatures drop and also the levels of light in water also decrease because of the low angle of the sun," states Ben Gratwicke. "These factors can all affect the way the fish looks. The colour of a carp is generally governed by pigment cells in the skin of the fish. These are called chromatophores and most are black but other colours do exist. These contract or expand to make a fish either lighter or darker respectively. This process is relulated by hormones in the fish's blood."

2 Carp camo!
In winter, the sun is generally lower in the sky and so this means that more light is reflected off the surface so less light penetrates into the water. Says Gratwicke: "So as far as the fish is concerned it is darker and it therefore adjusts its body colour to blend in with that." In carp angler chat it means you get a camouflaged carp.

3 Bloody hormones
The other factor is the process is regulated by some hormones which cause the chromatophores to contract. This makes the fish lighter in colour. "In winter however, these hormones are not being produced in a high level by the fish because of the cold temperatures and so the chromatophores expand causing the fish to appear darker. The colour doesn't generally depend on the water quality other than if it's stressing them, though lots of suspended solids tend to scatter more light and so make the background around the fish appear lighter near the surface."

4 Carp food
"The food they eat can affect the background colour of the skin and scales to some extent, but more often it is the pigments in the algae in the water they're in that has the most impact on colour. This is because they tend not to metabolise the pigment but store it instead," he concluded.