Is it normal for a Betta to be bright-colored or to have dull colors instead? I've read that wild Bettas usually look darker...but if a bright-colored Betta turns dark, it's a sign of disease/stress, right?
Very much depends on the type of betta. Koi and marble varieties will change color naturally as they age. Wild bettas tend to be more subdued, but will brighten up for courting/spawning behavior. All bettas can change color to some degree, and it can be associated with stress or boredom. Bettas can be very sulky--ie, water conditions are appropriate, temp is appropriate, but they don't have any stimulation, so they pout.