A. Your question is concerned with fish biology rather than stream ecology, and it would take considerable space to answer your question completely. Thus, I will give you some of the basic aspects of fish coloration; if you want to pursue this further, you can get more information from a good fish biology text.

A fish's iridescent hues are caused by crystals of guanin. On the belly, they appear in their undisguised silvery whiteness under the transparent skin. On the back, they are overshadowed by the thick black pigment which forms in that portion of the skin. Guanin crystals are waste products of the blood. Other colors are the result of one pigment - red, orange, yellow, or black - contained in individual sac-like structures called chromatophores. The amount of each pigment exposed is controlled by the chromatophore; that is why the coloration of fish can change. Different colored species have different amounts of the various pigments; their combination and distribution on the fish determine the characteristic patterns of each species. The blue and green colors are generally due to the reflections and interferences of light by the colorless surfaces of the guanin crystals and scales, acting in conjunction with the different pigment colorations. This is similar to the blue color in bird feathers; there is no blue pigment - it is the reflection of guanin crystals through the feather material.

The reason a fish loses its colors when it dies is because the pigment contracts within the chromatophore upon death, thus exposing less pigmentation and more of the colorless chromatophore.