As others have said there are other species in the catch all genus of Hyphessobrycon that look very similar to the serpae tetra, namely the red phantom tetra H. sweglesi. The rosy tetra and crystal red tetra also look very close. However, it is more likely that your fish are a variant of H. eques lacking the black ear, or more likely still, that they have not grown entirely comfortable, become well conditioned and established a heirarchy yet. These guys are aggressive in ways one would think of a piranha more so than any tetra.
As they mature and are fed high quality foods, yours is probably better than what they were getting if bought from a whole type supplier, the red will darken and they will get white trim on the fins. The ear spot will wash out when they are stressed so just give it some time and some good conditioning for them to reach their full potential. When fish are shipped they generally are put on a fast for a couple days so dont get discouraged just yet.
I got the impression you were more interested in the red coloration than the ear mark, so you should have no worries in a month or so when these guys are well fed and comfortable in their new surroundings. Just be careful with tankmates, these fish will run even larger peacful fish ragged.