Hi Jean-Pierre Collet.
I enjoy your pictures. I have seen that you use photoshop or another photo editing program to cut the borders of the animal and then past the animal in a blurred background. It is a nice effect.
On the color powder theme, Canthaxanthin and Apocarotinal (apo-8-carotenoic acid ethyl ester) are carotenoids, an important group of natural reddish/yellow pigments widely distributed in plants such as tomatoes, carrots or oranges. Many carotenoids are precursors of vitamin A. That means they develop certain vitamin A qualities in the body. This means they have a beneficial physiological value as well as enhancing colors or reviving pale and faded colors to their former beauty. And as Edward said it has to be used in the right amount and not for long periods of time. I normally use Necton Rep Color. You could dust it to the food or you could put it stright into the water. Amphibians living in water can absorb the vitamins through their skin or by swallowing the water. As a guide, it is recommended that you use a measuring-spoonful (1 gram) to 30 liters (about 7 gallons) of water. Or much less if you keep juveniles as bosca's newts in small aquarium with 2-7 cm of water. Because of the high concentration of the supplements, however, be careful when adding them to tanks containing plants; the trace elements could stunt plant growth.
I hope this answers your question
Best greetings
Yago