I would want to actually see this before I could give you a definite answer, as the behaviour might or might not be normal. But if the fish are severely stressed, then yes, this will cause aggression. Stress has many effects on fish, worsening as it increases or continues. My article on stress goes into all this: http://www.tropicalfishkeeping.com/f...um-fish-98852/
However, characins do establish hierarchies within their groups. And in fact, seeing increased interaction between the fish can be a good sign that the fish are suited to their environment. However, they will not be "washed out" but more intensely coloured, so this may be a clue that what you are observing in the store tank is not positive.
A couple weeks ago I acquired a group of wild-caught cardinals. After a couple days in their new home, their colours really intensified, and several of the males began what I refer to as "sparring." Pencilfish in the genus Nannostomus, which are also characins, do this a lot, and I see it from time to time among hatchetfish and tetra species. Many consider this "play," like young cats and dogs "playing," and while this is probably true I believe there is a much more serious aspect to this among the fish.