Colour changes in fish is quite normal, so don't worry about it. The only time you have to worry about colour changes is if they do change colour and go vary very pale and stay like that, this is when the fish maybe ill. However, you will see fish changing colour quite a lot, there are many factors that cause fish to change colour, aggression, breeding rituals, their surroundings, loads of reasons. The following information is from
www.newton.dep.anl.gov/natbltn/700-799/nb706.htm
. It's a little bit long winded but nevertheless, is very interesting and will give you a lot of information about why fish change colour
Fish of many kinds, on the other hand, change the pigmentation of their
skins to match the color of their background more or less closely. In the
1930's a series of experiments was carried out with the little Silver-
mouthed Minnow, a common fish of small streams and ditches in
central Illinois. Placed in a black-bottomed pan they became a dark
slate gray; in a white pan they faded to a pale straw color. When dipped
from the black to the white pan, or vice versa, the skin color changed
completely within two minutes. No matter how often it was repeated,
they changed as quickly as the first time. However, when they were
allowed to swim from a black background, or a white one, into a pan
painted half white and half black, 85 percent of them chose the
background to which they were already adapted.
The advantage to a fish of being able to match the color of its
background was proved by experiments with the little Mosquito Fish, or
Gambusia. For example, when fish from a white tank were added to an
equal number in a black tank, a penguin quickly devoured three times
as many of former as of the camouflaged dark ones. The value of this
protective coloration was shown still more strikingly in other tests when
a heron or a game fish was used as the predator.
The champion of all imitators is the Flounder, a fish which lies flat on
the bottom in shallow coastal waters. Not only does it adjust its skin
color through the various shades of gray but it accurately duplicates the
pattern of mud, sand, gravel, stone and shell bottoms. Checkerboard
and polka dot patterns are not imitated so exactly. Strangest of all,
black, off-white, blue, green, yellow, orange, pink and shades of brown
-- but not red -- are excellently matched. However, some of these
changes take a long time.
The color changes in all of these animals are, for the most part, the
result of changes in certain star-shaped pigment-bearing cells in the
skin, called chromatophores. Different ones contain granules of black,
yellow, orange or red pigment. In a dark-colored animal, for example,
the chromatophores with black pigment have the granules scattered
throughout the cell. When the skin fades these granules become
concentrated into a microscopic dot, leaving the remainder of the cell
colorless. This exposes chromatophores with other colors. Still deeper
in the skin are glistening silvery cells that reflect light like a mirror. The
control of these color changes involves the retina of the eye, nervous
system, the hormones in the blood, and so forth. The story becomes
very complicated.
