Clothing and diadem
When talking about Roman dress, the first thing that comes to mind is the toga, which we see on many ancient images like paintings, mosaics and statues, and which is commonly associated with the Roman elite. The simpler looking tunic, also seen on many images, was generally more casual and much more practical for day to day activities. The wearing of the toga was only reserved for men, and Roman women of Republican and early imperial Rome wore the stola. Helena is about a century too late for her dress to be called a stola, but I would like to argue that both dresses are very similar, and in her case more richly decorated.
The diadem she is wearing is also fitting for the period, and an interesting comparison can be made with Fausta, the wife of Constantine. Both women received the title of Augusta, but Helena is depicted more often with the diadem, more than any other female member of Constantine’s dynasty. Could this be a matter of status within the court? While this could certainly be the case, it could also simply be part of the hairstyle. There are depictions of Fausta wearing the diadem in which her hairstyle is identical to that of Helena on this coin. In this case the diadem was only part of the coiffure, and did not indicate any hierarchy within the dynasty.
It is interesting to imagine the transition from Helena’s clothing to the regal dress of the later Eastern Roman and Byzantine emperors and their courts. A good example would be the dress as depicted on a coin by the empress Irene from the late eighth century AD, where her diadem and her dress (the stemma and loros) do indeed look similar to the imperial clothing worn in the early fourth century, and which is depicted on the coin with Helena in the form of her diadem and stola.