IIRC, the biretta, for liturgical purposes, is only worn by clerics in Orders. It could be worn by seminarians not in orders as streetwear, but again IIRC, that would be unusual. the case presented is a Pontifical College in Rome, and for streetwear, the cappello romano (aka saturno) in simple, unadroned black would, I think, be more likely.
BTW, the pom is not necessarily a part of a biretta (at least those that are not part of a religious habit, and even then, it depends on the prescriptions of the Rule and/or Constitution). For example, in Brittany, the traditional biretta for secular clergy had a rather long, embroidered tassel instead of a pom.
Also, IIRC, clerical birettas in other than black are not worn except by hierarchs and members of certain Orders/Congregations where it is considered part of the habit. For example, the O.Praem (Norbertines) wear a white one, (to match the habit) and, again IIRC, at least some in the ICRSS (also Canons), including the Superior-General, wear a blue one (again to match the habit).