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.
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.