These are very interesting questions! I am
having trouble finding out why dogs' eyes change
color, but the basis for permanent color change in
human eyes may be similar.
In humans, melanocytes are cells that
produce pigments for eyes, hair, and skin. One of
the major pigments produced by such cells is
called melanin. As children develop, the
amount of melanin their melanocytes produce may
change over time, thus resulting in a change in
eye, hair, and skin color.
Fish and other organisms that change the colors
of their whole bodies, on the other hand, can
often do this in a reversible manner. Often, the
reason for this kind of color change is for
amouflage -- either as protection from
predators, or to trick prey into coming closer. I
hope this helps!
Fish can change color for a number of reasons,
and it depends on the fish. Fish containing
bioluminescent bacteria can make light (or rather,
the bacteria make light, and the fish just hosts
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