I'm not a fish expert. What I know has come from trial and error with poor Mr. Fishie and the intarwebz. You want to look for a fish that doesn't look depressed. You want one that is fairly active, and unless it's a mottled white coloring, you don't want one that is pale and sad looking. Some couple at the petsmart today wanted a bright red one, no matter what. They couldn't find one, and they looked kind of lost, so I suggested looking for a lively one in a coloration they liked. They complained none of them were red, and then said they didn't want one that was really excited because it would just jump out of the bowl like the last one and die.
I can't fault someone for being picky about color, but if you don't have a lid on your tank, a fish can jump out. More importantly.. if the fish is unhappy, he will jump out. If the water quality blows, he'll want to escape.
That being said, I found a very excited-to-be-here fish today, so the newly dubbed "Senior Pescado" came home with me today XD I was dead set on getting a blue crowntail, but when I saw him, I knew he had to come home with me. His coloration is totally gorgeous, and I'll have to take a photo later. He's a crowntail with a lot of different colors going on. His body is almost black, but he's got irridescent blueish-green going on up top. His fins start that great irridescent blue at the body, but they're red on the ends. The very tips of each fin are black. (And no, it doesn't look like fin rot lawl)
So he's got a lot going on, and he's really pretty. He didnt' look like much in the store because there wasn't any good light nearby to see him, and that gross blue water distorted the colors. He looked like he was blue with dark red fins, but I was pleasantly surprised when I got him home and in a clean tank of water that he's absolutely gorgeous.
I think it's also important to mention that as soon as I got the clean tank set up & Senior Pescado acclimated& happy, Senpu came up to the tank to check things out. I think Senpu is very happy with me right now, since he was so depressed after the passing of the original Mr. Fishie. He's got a new buddy now!