Fish tend to begin losing color immediately so reference pictures should be taken right away, at the water's edge.
Don't "play" your catch. Stressing the fish can not only change their colors, it also decreases their chance of surviving if you plan to release.
Don't use a stiff net. Nets can cause shredding of fins as it struggles. If you do net, try to grab the catch quickly and keep it from getting tangled. Be mindful of what kind of net you use, how, and when you use it.
Learn to use the "Macro" feature on your digital camera. This setting provides the best detailed reference pictures for your taxidermist. By magnifying the image larger than life-size, it enables the artist to see colors and patterns more clearly.
Shots to include: Take full pictures of both sides of your fish, the back from above, and belly to help the taxidermist visualize the natural variations no matter which way you choose to mount it. Then take close-up shots in three sections; head (mouth open/mouth closed), mid-section, and tail. Bigger fish may require more sectional shots. The more pictures and angels, the better.
Details: Was you fish caught before and leaving a battle wound on it's lip? Is his fin worn thin, or eye shape slightly different? All these details should be documented so your taxidermist can incorporate them in your mount.
Try to keep your fingers out of the shot. But how do I hold onto a struggling fish? you may ask ... Trick - hold the fish upside down to disorient it a little -- may not be the trophy shot you want to put in your scrapbook - but your taxidermist doesn't care.
Lighting. Full sun -- keep the sun to your back and try to get straight on shots to avoid shadows. Shade - avoid speckled shade, but if you can shade the catch from full reflective sun, this often produces much better color. Cloudy days are probably the best - but if the light is too low, you may need to use a flash - again trying to avoid shadows.
Quick tip for releasing your fish - put them back in the water, holding them gently by the tail. Pull them back slowly to push water over the gills. This gives them an extra shot of oxygen and helps them to revive and orient themselves. Tossing them through the air to splashdown is not only rough on their senses, but can actually damage fragile scales and fins and hurt their chances of survival.