Guppies do feel and suffer from loneliness. I realized that when I put a sick guppy in a hospital tank alone, it tends to die within a few days. However, when I have two or more guppies, they can live for weeks. During that time, they can be treated.
Yes, guppies can be trained. Guppies can be trained to gather at the side of the glass when it's feeding time. Guppies will be able to learn that when they see a hand over the tank, it means that food will be dropped into the tank. They will also follow one's hand around the tank. Guppies also tend to stay around the area where they are usually fed.
Guppies do like green more than most other colours. Wild guppies live in a green environment. Therefore, they feel at home in a green environment. Guppies in aquariums kept their natural instinct of liking green.
This is a question that has never been solved by scientists. I think that guppies do feel pain because they avoid things that hurt them, such as violent fishes and nets. When a guppy's fins get nipped by another fish, the guppy doesn't stay there, it swims away, fast. Why? Because it felt the pain dealt by the other fish.
Guppies do have a sense of taste because they like certain foods more than others. Guppies tend to like meats such as bugs, worms, red meat, and white meat more than grains. Guppies also detest medicated food.
Salt will make the water slightly clearer by killing algae. When salt is added to water with algae, water inside the algae will be drawn out of it due to osmosis. This will make the algae die of dehydration. However, this will have a significant effect because you can't use too much salt; otherwise, your guppies will die as well.